base-files: Ensure reset only works if an overlay exists

Currently the reset script will try to run jffs2reset on boards that are
running a rw rootfs, such as ext4. This will cause jffs2reset to fail
and the board to never reboot while the LED blinks until a manual
reboot.

This commit does two different things:
1. Disables reset on boards that do not have an overlay mount
2. Disables the Blinking LED after 5 seconds if the board does not
support reset

Signed-off-by: Chris Blake <chrisrblake93@gmail.com>