buildroot: allow enabling MIPS16 user-space build

Enabling MIPS16 is made conditional on advertising the "mips16" feature
for a specific target since it requires support from the CPU
(HAS_MIPS16) and the actual use of MIPS16 for building packages
(USE_MIPS16).

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36202 3c298f89-4303-0410-b956-a3cf2f4a3e73