gen_gerrit_meta.sh: Use $GERRIT_CHANGE_NUMBER if available

Different jobs seem to pass Gerrit info in different ways, so add an
alternative code path using GERRIT_CHANGE_NUMBER/GERRIT_PATCHSET_NUMBER.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ie14504c5fec518d0e690afb3acddaff0185def75
1 file changed