Build instructions feedback for macOS

[ole]

I'm a hobbyist and learned about Hubris on Twitter. I wanted to give a brief report about my experience getting the demo app to run on an STM32F439ZI board on an M1 Mac running macOS 12.0.1.

1. Despite no official macOS support, the installation instructions in the readme worked fairly well for me (kudos!). (Worth noting that I had already installed Rust and all required tools because I have dabbled with embedded Rust and used other arm-none-eabi-gcc-based toolchains before.)

   I used this arm-none-eabi-gcc variant because that's the one I have installed:

   brew tap ArmMbed/homebrew-formulae
   brew install arm-none-eabi-gcc

   If you're on an M1/Apple Silicon Mac, it's worth noting that arm-none-eabi-gcc is not yet available natively AFAIK, but you can install the Intel/x86 version and run it through Rosetta. From an arm64 terminal, run this command to open an x86 shell in Rosetta:

   arch -arch x86_64 zsh

   In this shell, you can then install Homebrew for x86 (will install in a different directory than arm64-Homebrew) and then proceed to install arm-none-eabi-gcc. See e.g. this article for more on keeping two arm64 and x86 versions of Homebrew side by side.

2. When it came to building the demo app, I at first missed the note in the readme to clone all submodules. This cost me 30 minutes. Totally my fault, but maybe you can make this clearer by making this step a bullet point in the Prereqs or Build section instead of tacking it on at the bottom of the Build section. I read the readme from the top and didn't even consider reading further when the cargo xtask dist app/demo-stm32f4-discovery/app.toml command failed because of a missing file.

3. Building succeeds, but I'm seeing one warning when building hiffy. Is this a problem with my setup or to be expected?

   warning: task-hiffy v0.1.0 (/Users/elo/hubris/task/hiffy) ignoring invalid dependency `drv-stm32fx-rcc` which is missing a lib target

   Here's the full output of the build command

   $ cargo xtask dist app/demo-stm32f4-discovery/app.toml
       Finished dev [optimized + debuginfo] target(s) in 0.08s
       Running `target/debug/xtask dist app/demo-stm32f4-discovery/app.toml`
   flash = 8000000..8040000
   ram = 20000000..2001c000
   Used:
   flash: 0x2e100
   ram: 0x8000
   building path app/demo-stm32f4-discovery/../../task/jefe
       Finished release [optimized + debuginfo] target(s) in 0.07s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/task-jefe -> target/demo-stm32f4-discovery/dist/jefe
   building path app/demo-stm32f4-discovery/../../drv/stm32fx-rcc
       Finished release [optimized + debuginfo] target(s) in 0.06s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/drv-stm32fx-rcc -> target/demo-stm32f4-discovery/dist/rcc_driver
   building path app/demo-stm32f4-discovery/../../drv/stm32fx-usart
       Finished release [optimized + debuginfo] target(s) in 0.06s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/drv-stm32fx-usart -> target/demo-stm32f4-discovery/dist/usart_driver
   building path app/demo-stm32f4-discovery/../../drv/user-leds
       Finished release [optimized + debuginfo] target(s) in 0.07s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/drv-user-leds -> target/demo-stm32f4-discovery/dist/user_leds
   building path app/demo-stm32f4-discovery/../../task/ping
       Finished release [optimized + debuginfo] target(s) in 0.06s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/task-ping -> target/demo-stm32f4-discovery/dist/ping
   building path app/demo-stm32f4-discovery/../../task/pong
       Finished release [optimized + debuginfo] target(s) in 0.06s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/task-pong -> target/demo-stm32f4-discovery/dist/pong
   building path app/demo-stm32f4-discovery/../../task/hiffy
   warning: task-hiffy v0.1.0 (/Users/elo/hubris/task/hiffy) ignoring invalid dependency `drv-stm32fx-rcc` which is missing a lib target
       Finished release [optimized + debuginfo] target(s) in 0.07s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/task-hiffy -> target/demo-stm32f4-discovery/dist/hiffy
   building path app/demo-stm32f4-discovery/../../task/idle
       Finished release [optimized + debuginfo] target(s) in 0.06s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/task-idle -> target/demo-stm32f4-discovery/dist/idle
   building path app/demo-stm32f4-discovery/.
       Finished release [optimized + debuginfo] target(s) in 0.07s
   /Users/elo/hubris/target/thumbv7em-none-eabihf/release/demo-stm32f4-discovery -> target/demo-stm32f4-discovery/dist/kernel

4. Next step, running the demo app on the board. This command from the Run section in the readme didn't work for me:

   cargo xtask gdb app/demo-stm32f4-discovery/app.toml openocd.gdb

   Because it didn't find openocd.gdb (I ran this from the repository root because that's what the path to app.toml indicates). I had to specify the path to openocd.gdb:

   cargo xtask gdb app/demo-stm32f4-discovery/app.toml app/demo-stm32f4-discovery/openocd.gdb