fix usage of ADK_TARGET_ARCH

mk/build.mk

@@ -421,7 +421,7 @@ endif
 	fi
 	@if [ ! -z "$(ADK_TARGET_ARCH)" ];then \
 		grep "^config" target/config/Config.in.arch.choice \
-			|grep -i "$(ADK_TARGET_ARCH)"\$$ \
+			|grep -i "$(ADK_TARGET_ARCH)$$" \
 			|sed -e "s#^config \(.*\)#\1=y#" \
 			 >> $(ADK_TOPDIR)/.defconfig; \
 	fi