build: clean up stale files from a previous build when installing a package build to the staging dir

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

Backport of r36755

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@36916 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: backport libjson-c, libubox and uci from trunk

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

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@36917 3c298f89-4303-0410-b956-a3cf2f4a3e73