Unify alpine and non-alpine builds. The only difference is foreign
arch builds need some preparation -- separate that to a dependency.

Unfortunately we can't use wildcard targets ("%: ") as non-wildcard
ones are prevaling. Therefore, a somewhat ugly hack to generate
$arch: Dockerfile.$arch dependency is added.

While at it:
 - rename DB_ARGS to DB_CC
 - mark clean as phony
 - make "all" really run all the builds we can

Nice side effect: autocompletion ("make -C scripts/build <TAB>")
now works!
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>