fs_mgr: overlayfs support legacy devices (marlin)

Expand the tests to deal with the boot environment for marlin.
Recognize that older overlayfs drivers do not report to /sys/module
and the parsing /proc/filesystem is another place to interrogate this.

Test: system/core/fs_mgr/tests/adb-remount-test.sh
      (kernel w/ and w/o overlayfs)
Bug: 120448575
Bug: 123079041
Change-Id: I5fc5f01b4e4788ac57541cb5235f7ac4e4284d71
1 file changed