allow to override instruction set

mk/build.mk

@@ -421,6 +421,12 @@ defconfig: .menu $(CONFIG)/conf
 			|sed -e "s#^config \(.*\)#\1=y#" \
 			>> $(ADK_TOPDIR)/.defconfig; \
 	fi
+	@if [ ! -z "$(ADK_TARGET_INSTRUCTION_SET)" ];then \
+		grep "^config" target/config/Config.in.archopts \
+			|grep -i "$(ADK_TARGET_INSTRUCTION_SET)$$" \
+			|sed -e "s#^config \(.*\)#\1=y#" \
+			>> $(ADK_TOPDIR)/.defconfig; \
+	fi
 	@if [ ! -z "$(ADK_TARGET_ENDIAN)" ];then \
 		grep "^config" target/config/Config.in.endian \
 			|grep -i "$(ADK_TARGET_ENDIAN)" \