platform2: pass args down to compile steps

This let's you run:
	./platform2.py --board=x86-alex ... --action=compile crash_reporter
And it'll only build crash_reporter.

BUG=chromium:337753
TEST=`cbuildbot chromiumos-sdk` passes
TEST=running platform2.py by hand with specific targets works

Change-Id: Ibad55a67cc5df9fb8b2c18c00e6e4ca7787f1f18
Reviewed-on: https://chromium-review.googlesource.com/183654
Reviewed-by: Liam McLoughlin <lmcloughlin@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed