build: reimplement package build directory override

Remove leftover pieces from package-version-override.mk support
Add a new variable USE_SOURCE_DIR to use a custom build directory.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40527 3c298f89-4303-0410-b956-a3cf2f4a3e73