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>

git-svn-id: svn://svn.openwrt.org/openwrt@36755 3c298f89-4303-0410-b956-a3cf2f4a3e73
libubox: update to latest version, adds support for the new json-c and adds a new blobmsg_json helper

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

git-svn-id: svn://svn.openwrt.org/openwrt@36756 3c298f89-4303-0410-b956-a3cf2f4a3e73