build: rework language dependencies

build: rework language dependencies

Rework the language package dependencies to avoid introducing unnecessary
depends. The luci-i18n-$app-$lang packages now depend on luci-$app.

 * When a language is selected as <*> then the language packs for each
   enabled LuCI module are built into the image
 * When a language is selected as <m> then the language packs for each
   enabled LuCI module are built as package
 * When CONFIG_ALL is set (e.g. release builds, snapshots) then all
   language packs are built

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>