Correct ih_os for u-boot.img

Provide appropriate '-O u-boot' while doing mkimage
for u-boot.img

Signed-off-by: Aneesh V <aneesh@ti.com>

Makefile

@@ -375,7 +375,7 @@ $(obj)u-boot.ldr.srec:	$(obj)u-boot.ldr
 
 $(obj)u-boot.img:	$(obj)u-boot.bin
 		$(obj)tools/mkimage -A $(ARCH) -T firmware -C none \
-		-a $(CONFIG_SYS_TEXT_BASE) -e 0 \
+		-O u-boot -a $(CONFIG_SYS_TEXT_BASE) -e 0 \
 		-n $(shell sed -n -e 's/.*U_BOOT_VERSION//p' $(VERSION_FILE) | \
 			sed -e 's/"[	 ]*$$/ for $(BOARD) board"/') \
 		-d $< $@