Add variables to support checking system manifest.

System manifest is determined when all of /system/etc/vintf/manifest*
are built. Export a few variables to support checking
system manifest against frozen data.

Bug: 37226359
Test: freeze.sh 5 && m check-vintf-all

Change-Id: I9906e888e3cb7d14851a98526adf02706c4ed08b
1 file changed