configuregateway and it's gateway.d files represent a specific
functionality that other packages depend on. Thus, it is put into
a package of its own so dependencies can be expressed more properly.
While at, use the chance to get rid of the ambiguous term "gateway"
and rename the script to configure-layer3 and the folders to layer3.d.
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Christian Dresel <freifunk@dresel.systems>
