lpmake: Allow writing split super images.

To use this mode, the build must be a retrofit build and --output should
be a folder rather than a file.

This also fixes a bug in lpdump where it refuses to dump full super.img
images on the host.

Bug: 118887112
Test: manual test
Change-Id: I3318684fbea6fe67bfa171001beea3a5c560fbab
2 files changed