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>