Document how to use cross-toolchain-helper with the SDK

docs/04-Cross-compilation-inside-the-SDK.md

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+## Cross-compiling inside the SDK
+
+To cross-compile WebKit for different machines, WebKit provides a yocto based environment.
+See /path/to/WebKit/Tools/yocto/README.md for details. You do not need to cross-compile from
+within the SDK, but if you want to do it, here are some instructions:
+
+```
+unset LD_LIBRARY_PATH
+export NUMBER_OF_PROCESSORS=12 # Adapt to your machine.
+export WEBKIT_USE_SCCACHE=0
+Tools/Scripts/cross-toolchain-helper --cross-target rpi3-32bits-mesa --build-image
+Tools/Scripts/cross-toolchain-helper --cross-target rpi3-32bits-mesa --build-toolchain
+Tools/Scripts/cross-toolchain-helper --cross-target rpi3-32bits-mesa --cross-toolchain-run-cmd CFLAGS="" CPPFLAGS="" WebKitBuild/CrossToolChains/rpi3-32bits-mesa/build/toolchain/sysroots/x86_64-pokysdk-linux/post-relocate-setup.d/meson-setup.py
+Tools/Scripts/cross-toolchain-helper --cross-target rpi3-32bits-mesa --cross-toolchain-run-cmd Tools/Scripts/build-webkit --wpe --release
+```
+
+It is important to unset the `LD_LIBRARY_PATH` otherwise `cross-toolchain-helper` will partly fail,
+but `build-webkit` continues, leading to an inconsistent build environment, that will fail to produce binaries.
+
+sccache is also not supported in that mode, and will interefer with yocto - disable it.
+
+Follow the instructions in Tools/yocto/README.md to flash the image onto your target machine.