build: add a variable for the name of the {build,staging}_dir/{target,toolchain}-* directories

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39857 3c298f89-4303-0410-b956-a3cf2f4a3e73