Add the LLVM Embedded Toolchain for Arm to the full image.

While being mostly about Cortex-M devices, the LLVM Embedded Toolchain for Arm (https://github.com/ARM-software/LLVM-embedded-toolchain-for-Arm) provides an LLVM based bare-metal toolchain with support for aarch64. This is useful when one want to experiment in a lightweight fashion with new architectural features for example which do not require a full software stack, but still benefit from a baremetal toolchain with a standard library and semi-hosting support.

As the executables may conflict with other llvm / clang installed binaries on the search path, the LLVM_ET is not added to the search path. Instead, the environmet variable TCH_LLVM_ET_INSTALL_DIR is provided and the user should use it in his build scripts.

Signed-off-by: Arnaud de Grandmaison arnaud.degrandmaison@arm.com