Skip running arch/arm/mach-imx flash.bin generation in case BINMAN is
enabled, otherwise the target in arch/arm/mach-imx/Makefile regenerates
the flash.bin again and produces corrupted result.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Peng Fan <peng.fan@oss.nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>