build: Use $(MAKE) instead of direct call to make

No functional changes, just make it to respect original make binary
name used to start building

Type: improvement

Change-Id: Ic8568237fbb39c6a0d3b7405a9670e9410aeb752
Signed-off-by: Renato Botelho do Couto <renato@netgate.com>

build-root/Makefile

@@ -997,7 +997,7 @@ $(PLATFORM_IMAGE_DIR)/ro.img ro-image: $(patsubst %,%-find-source,$(ROOT_PACKAGE
 		| xargs sign $($(PLATFORM)_public_key)			\
 			     $($(PLATFORM)_private_key_passphrase) ;	\
 	  fi ;								\
-	  : make read-only file system ;				\
+	  : $(MAKE) read-only file system ;				\
 	  mksquashfs							\
 	    $${tmp_dir} $${ro_image}					\
 	    -no-exports -no-progress -no-recovery ;			\