fs_mgr: overlayfs: test issues with single super device

Cleanup test issues with single super device.  Indicate mount point
instead of partitions as that can lead to confusion when referencing
the partition when there is none.  For example root (/) partition
uses overlay on /system.  Resolve a script syntax error when there
are no devices.  Fix script logic surrounding detection of overlayfs
takeover and handling of reboot device request from
adb disable-verity.

In README.overlayfs.md discuss
BOARD_<partitiion>IMAGE_PARTITION_RESERVED_SIZE and its negative
impact on right sizing.

Soften README.overlayfs.md wording with respect to update_engine
to match changes in handling.

Test: adb-remount-test.sh
Bug: 120536582
Bug: 109821005
Change-Id: Iad7e19a436322ff9c3bdc597f4545028be112ff5