assemble_vintf: getFlagIfUnset harder errors

assemble_vintf fails if, for BOARD_SEPOLICY_VERS, POLICYVERS and
- If the environment variable is set and the value has an unknown
  format; or
- If the environment variable is set and it conflicts with an
  existing value (e.g. hard-coded in a device manifest file)

Remove spurious warnings for undefined environment variables since
the manifest / matrix merge logic merge these values from fragments;
it is okay for these values to be undefined in fragments.

Fixes: 116614862

Test: cd hardware/interfaces/health/storage/1.0/default && mma
Test: builds

Change-Id: I387cedf03ff0c8f703507f4a95e5807eb03b65d2
1 file changed