Add the LLVM Embedded Toolchain 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 that provides 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 environment variable TCH_LLVM_ET_PATH
is provided and the user should use it in his build scripts.

I had to pass RT at starmap time as it seems that on my Mac at least
global variables are not shared amongst processes and I was getting 'None'
for IMAGE and RUNTIME. Those 2 have been merged into a single RT global
variable to avoid duplication and risks of mismatches.

Misc fixes in genassets.sh while being there.

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