include: Add nomips16 CPU_SUBTYPE

This can be used to indicate that a target does not support the optional mips16
extension even when it is a mips32r2 or later CPU.

This will generate a separate toolchain and a separate package folder,
e.g. mips_24kc_nomips16

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>

include/target.mk

@@ -258,8 +258,10 @@ ifeq ($(DUMP),1)
       FEATURES += virtio
     endif
     ifneq ($(CONFIG_CPU_MIPS32_R2),)
+      ifneq ($(CPU_SUBTYPE),nomips16)
         FEATURES += mips16
       endif
+    endif
     FEATURES += $(foreach v,6 7,$(if $(CONFIG_CPU_V$(v)),arm_v$(v)))
 
     # remove duplicates