Allow Mbed CE to be built as an Arduino core

[multiplemonomials]

Summary of changes

This PR works together with the new https://github.com/mbed-ce/ArduinoCore-mbed-ce repository. Using this PR plus that repo, one can compile an Arduino IDE compatible core for Mbed CE! All you gotta do is drop the core folder in the $HOME/Documents/Arduino/hardware/mbed-ce folder, and bam! The Arduino IDE can then compile for supported boards (currently Nano 33 BLE and RPi Pico) using Mbed CE as a backend.

Many of the patches needed for these targets already existed in Mbed from earlier efforts to keep the Arduino targets in sync, but some more still had to be added. I tried to not go completely bonkers adding changes, and just go for the minimum needed to make it compile and work.

Changes include:

• Move mbed-target-config.h to a subfolder of CMAKE_BINARY_DIR. This makes it easier to include without picking up all of Mbed OS!
• Merge Arduino patch which allows loading TLS certificates from the filesystem for a TLS socket. This is, I believe, more efficient than reading the file into RAM because it accesses the file directly.
• Add mbedtls.entropy-nv-seed which allows MBEDTLS_ENTROPY_NV_SEED to be enabled using target_overrides (instead of being forced to enable it globally using macros)
• Merge Arduino patch to change I2CSlave::read() return value.
• Don't download greentea client if greentea testing isn't actually enabled. Why wasn't it like this before? 🤷‍♂️
• Include mbed-target-config.h from mbed.h. This means that code external to mbed, which includes mbed.h, no longer has to have the -include mbed-target-config.h flag set up, which is convenient.
• Fix SDBlockDevice compile error introduced by async SPI change
• Fix adc.c missing from RPi Pico source file list, meaning that any attempt to use the ADC would cause a linker error
• Add hardware_pio and hardware_dma libraries for RPi Pico
• Update RPi Pico SDK importer to use regex matching. This means it won't erroneously rename functions like adc_gpio_init(). (because before it was just replacing any instance of gpio_init). The import is slower now but only I have to deal with that...
• ARDUINO_NANO33BLE_SWD now inherits from ARDUINO_NANO33BLE (so that the correct target label is applied even in SWD mode).
• mbed-target-config.h now contains an include guard.
• The Mbed toolchain file now sets C11 and C++17 as the default language standards.

Impact of changes

• Mbed CE can be used as an Arduino core!
• New mbedtls.entropy-nv-seed option which enables the MBEDTLS_ENTROPY_NV_SEED define

Migration actions required

• I2CSlave::read() now returns the number of bytes actually read. This is pretty much required in order to know how many bytes were actually sent by the master -- I don't know why it wasn't like this before!! Honestly I would be surprised if anybody was actually using this API given how broken it was.

Documentation

See ArduinoCore-mbed-ce repo.

---

Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[X] Major update (Breaking change E.g. Return code change / API behaviour change)

---

Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

I've build and run several examples, and they work!

---

Reviewers

---