scripts: build-all.py: append llvm to the config file

To detect that the Kernel is being compiled with llvm, append llvm
to the config file.

Change-Id: Iba2f4f83f5ab091c7906dc66ad11cc88bcdfe2b9
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>