util: Change gen_arm_fs_files.py to allow selective compilation

util: Change gen_arm_fs_files.py to allow selective compilation

With the -b/--fs-binaries option it is possible to specify a list
of fs binaries to be fetched/compiled.

Change-Id: I12a642f65b74e8606c82cdddcbc3a8172bad2381
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32554
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>