uboot-at91: make packages hidden

These packages are needed to generate the image, better mark them hidden
so we will activate them based on which boards gets build and they will
be activated always when the board which needs then gets build.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/boot/uboot-at91/Makefile

@@ -17,6 +17,7 @@ include $(INCLUDE_DIR)/package.mk
 
 define U-Boot/Default
   BUILD_TARGET:=at91
+  HIDDEN:=1
   UBOOT_IMAGE:=u-boot.bin boot.bin
 endef