build: move the -j flag to package.mk instead of grabbing it from makeflags,...

build: move the -j flag to package.mk instead of grabbing it from makeflags, otherwise it occasionally gets omitted on full builds

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@33422 3c298f89-4303-0410-b956-a3cf2f4a3e73