As we have base_system_ext.mk for the packages in system_ext
partition, move the group and passwd files for system_ext partition
to base_system_ext.mk.
Bug: 141658360
Test: mainline checks pass
Change-Id: Iff50482b90ff9877a58c54c8149014e8c103a3c6
The default packages for system_ext partition must be added to
*_system_ext.mk.
We have empty list for now, but will update the files soon.
Bug: 134359158
Bug: 139053989
Test: build and boot
Change-Id: I78c85c3a9ff0437906143c2d4982ecfb0e4d058d