build: Ensure /system unmounted at install start

Account for the possibility that a user has mounted system manually
before installing the FullOTA package.

Change-Id: I97d0540676a7ce98762545e76ecb1a7f6335d204
(cherry picked from commit bc8ed1337ad8975c0c2483a48c004991eb6c2295)
repopick: handle variant hal branching

Change-Id: Ib0dee19abc13a7fb8d8e42c09b73c1b4f35ca938
(cherry picked from commit 6be14313c78013dcd5f562f73745ab6b744cca50)
releasetools: ota_from_target_files: add FullOTA_PostValidate

Change-Id: I152412049f90fd546d4516cc064238c3192be553
(cherry picked from commit c7a9bac0015d34e16510f978e24784e25b86aaa6)
Add ext2, ext3, and vfat to releasetools

Support some non-standard partition that may be found in recovery.fstab

Conflicts:
	tools/releasetools/common.py

Change-Id: I0026a1d3ae5d432db10150d1a15f7bc8e7a6054b
(cherry picked from commit 982f4f7809be4ff33044148a429af0c9d4d40f6a)