Makfile: fix a rule to build u-boot.sb

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

Makefile

@@ -490,7 +490,7 @@ $(obj)u-boot.ais:       $(obj)spl/u-boot-spl.bin $(obj)u-boot.img
 
 
 $(obj)u-boot.sb:       $(obj)u-boot.bin $(obj)spl/u-boot-spl.bin
-		$(MAKE) -C $(SRCTREE)/$(CPUDIR)/$(SOC)/ $(OBJTREE)/u-boot.sb
+		$(MAKE) $(build) $(SRCTREE)/$(CPUDIR)/$(SOC)/ $(OBJTREE)/u-boot.sb
 
 # On x600 (SPEAr600) U-Boot is appended to U-Boot SPL.
 # Both images are created using mkimage (crc etc), so that the ROM