generate_build_files.py is a generalisation of update_gypi_and_asm.py
that works for both Chromium and Android. In the future, these projects
will drop their copies of update_gypi_and_asm.py and will use this
script instead.

Change-Id: Ie32ce629ef44a6c070e329c7bc5e4531205b9709
Reviewed-on: https://boringssl-review.googlesource.com/4631
Reviewed-by: Adam Langley <agl@google.com>