uboot: fix custom patch dir typo

Noticed by robinmholt on IRC.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

boot/uboot/uboot.mk

@@ -56,7 +56,7 @@ define UBOOT_APPLY_CUSTOM_PATCHES
 		uboot-$(UBOOT_VERSION)-\*.patch
 endef
 
-UBOOT_POST_PATCH_HOOKS += U_BOOT_APPLY_CUSTOM_PATCHES
+UBOOT_POST_PATCH_HOOKS += UBOOT_APPLY_CUSTOM_PATCHES
 endif
 
 define UBOOT_CONFIGURE_CMDS