fdtoverlay: Return non-zero exit code if overlays can't be applied

At present the tool terminates its execution if one of the overlays passed
as command-line arguments can't be successfully read or applied, but the exit
code of the process is zero, making failures hard to detect inside scripts.

Signed-off-by: Valter Minute <valter.minute@toradex.com>
Message-Id: <20191009123256.14248-1-valter.minute@toradex.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>