build: add mkrasimage

build: add mkrasimage

The current make-ras.sh image generation script for the ZyXEL NBG6617
has portability issues with bash. Because of this, factory images are
currently not built correctly by the OpenWRT buildbots.

This commit replaces the make-ras.sh by C-written mkrasimage.

The new mkrasimage is also compatible with other ZyXEL devices using
the ras image-format.
This is not tested with the NBG6616 but it correctly builds the
header for ZyXEL factory image.

Signed-off-by: David Bauer <mail@david-bauer.net>
(backported from 8e9a59a6b924469c0061d4bd945ae0c6dbfdecfe)