uboot: unbreak kirkwood format handling after a4ca959321

Commit a4ca959321 (u-boot: rename to uboot) forgot to change the kirkwood
format handling to look for the new variable name, breaking support.

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

boot/uboot/uboot.mk

@@ -24,7 +24,7 @@ endif
 
 ifeq ($(BR2_TARGET_UBOOT_FORMAT_KWB),y)
 UBOOT_BIN          = u-boot.kwb
-UBOOT_MAKE_TARGET  = $(U_BOOT_BIN)
+UBOOT_MAKE_TARGET  = $(UBOOT_BIN)
 else ifeq ($(BR2_TARGET_UBOOT_FORMAT_LDR),y)
 UBOOT_BIN          = u-boot.ldr
 else