ARM: keystone2: Build MLO by default

MLO(NAND/MMC boot image), is used for all the ks2 platforms.
Enabling it in config.mk so that these images will be automatically
built upon calling make. u-boot-spi.gph is already the build target,
so not including here.

Reported-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>

arch/arm/mach-keystone/config.mk

@@ -5,6 +5,10 @@
 # SPDX-License-Identifier:     GPL-2.0+
 #
 
+ifndef CONFIG_SPL_BUILD
+ALL-y += MLO
+endif
+
 MKIMAGEFLAGS_u-boot-spl.gph = -A $(ARCH) -T gpimage -C none \
 	-a $(CONFIG_SPL_TEXT_BASE) -e $(CONFIG_SPL_TEXT_BASE) -n SPL
 spl/u-boot-spl.gph: spl/u-boot-spl.bin FORCE