treewide: move MAC address patch functions to common library

treewide: move MAC address patch functions to common library

This unifies MAC address patch functions and moves them to a
common script. While those were implemented differently for
different targets, they all seem to do the same. The number of
different variants is significantly reduced by this patch.

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