mvebu: support UART boot image

The kwboot utility can use the generated image to boot mvebu SoCs from
UART.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/arch/arm/mach-mvebu/Makefile b/arch/arm/mach-mvebu/Makefile
index d907fac..ee2eca9 100644
--- a/arch/arm/mach-mvebu/Makefile
+++ b/arch/arm/mach-mvebu/Makefile
@@ -35,6 +35,9 @@
 ifneq ($(CONFIG_MVEBU_SPL_BOOT_DEVICE_MMC),)
 	KWB_CFG_BOOT_FROM=sdio
 endif
+ifneq ($(CONFIG_MVEBU_SPL_BOOT_DEVICE_UART),)
+	KWB_CFG_BOOT_FROM=uart
+endif
 
 ifneq ($(CONFIG_SECURED_MODE_IMAGE),)
 KWB_REPLACE += CSK_INDEX