luci.mk: reorder to have common-things in a block

The luci.mk file has grown over the years and became more complex. This
change reorders some sections to make it more easy to read and understand
on the first view.

* move all constants into a common block
* put all standard package definitons together
* put the specific sections for the "luci-base" behind the common things
* have the defines for the translations at the very end

Signed-off-by: Sven Roederer <freifunk@it-solutions.geroedel.de>