Fix running platform2.py outside of an emerge

BUG=chromium:220003
TEST=Build Platform2 host/target using emerge and platform2.py

Change-Id: Ifcc5429aa02229a0fe4297d0fd65adc58f666e63
Reviewed-on: https://gerrit.chromium.org/gerrit/56872
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Liam McLoughlin <lmcloughlin@chromium.org>
Commit-Queue: Liam McLoughlin <lmcloughlin@chromium.org>