ARM: add u-boot.imx as target for i.MX SOCs

Freescale SOCs require an header to u-boot.bin

The patch adds u-boot.imx to the default targets
if the imx file is set (IMX_CONFIG).

Signed-off-by: Stefano Babic <sbabic@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
CC: Loïc Minier <loic.minier@linaro.org>
CC: Mike Frysinger <vapier@gentoo.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>
Tested-by: Dirk Behme <dirk.behme@googlemail.com>

arch/arm/cpu/armv7/config.mk

@@ -31,3 +31,6 @@ PLATFORM_CPPFLAGS += -march=armv5
 # =========================================================================
 PF_RELFLAGS_SLB_AT := $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
 PLATFORM_RELFLAGS += $(PF_RELFLAGS_SLB_AT)
+ifneq ($(CONFIG_IMX_CONFIG),)
+ALL-y	+= $(obj)u-boot.imx
+endif