Add CONFIG_OVERLAY_FS=y

Required for overlayfs integration support for adb remount
command.  A requirement for any device that supports a
readonly filesystem, which includes erofs, squashfs, ext4 dedupe,
or right sized (zero space remaining) for any filesystem type
selected for the system images.

Required for vendor overlays.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Test: adb-remount-test.sh
Bug: 109821005
Change-Id: I3ece03886db7cc97f864497cf93ec6c6c39bccd1