lantiq: move 01_led_migration script to subtargets

lantiq: move 01_led_migration script to subtargets

Split the /etc/uci-defaults/01_led_migration scripts into subtargets
as already done for most of the other base-files.

While this introduces a minor amount of code duplication, it still
is considered an improvement, as device-specific settings are kept
together in the subtargets' base-files and the script at hand can be
removed entirely for two of the subtargets not needing it.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>