lantiq: bring back okli loader

Removed due to being unused with 1f7a03a7, but now required for the
ar7 FRITZ!Box.

Could be used for the ARV7519RW22 as well, for which the image
generation was disabled due to a stock u-boot issue with kernel bigger
than 2 MByte.

The code is combination of the ath79 and ramips okli loader.

Signed-off-by: Mathias Kresin <dev@kresin.me>