Fix dependency for custom patches when custom patch is empty.

target/linux/Makefile.in.advanced

@@ -71,7 +71,9 @@ LINUX26_PATCH_SITE:=$(strip $(subst ",,$(BR2_KERNEL_PATCH_SITE)))
 LINUX26_BSP_PATCH:=$(strip $(subst ",,$(BR2_LINUX_BSP_PATCH)))
 #"))
 
+ifneq ($(LINUX26_BSP_PATCH),)
 LINUX_HEADERS_DEPENDS:=  $(DL_DIR)/$(LINUX26_BSP_PATCH)
+endif
 
 ifneq ($(LINUX26_PATCH_SOURCE),)
 LINUX_HEADERS_DEPENDS+=$(DL_DIR)/$(LINUX26_PATCH_SOURCE)