sdk: keep $(STAGING_DIR)/usr/bin, at least curl needs it

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

Backport of r42397

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42398 3c298f89-4303-0410-b956-a3cf2f4a3e73
Merge branch 'master' of git://git.openwrt.org/14.07/openwrt