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

@@ -258,7 +258,9 @@ ifeq ($(DUMP),1)
       FEATURES += virtio
     endif
     ifneq ($(CONFIG_CPU_MIPS32_R2),)
-      FEATURES += mips16
+      ifneq ($(CPU_SUBTYPE),nomips16)
+        FEATURES += mips16
+      endif
     endif
     FEATURES += $(foreach v,6 7,$(if $(CONFIG_CPU_V$(v)),arm_v$(v)))