Implement BOOT_OFFSET command for imximage. This command is parallel
to current BOOT_FROM command, but allows more flexibility in configuring
arbitrary image header offset. Also add an imximage.cfg with default
offset values into arm/arch/imx-common/ so the board-specific imximage.cfg
can include this file to avoid magic constants.

The syntax of BOOT_OFFSET command is "BOOT_OFFSET <u32 offset>".
Signed-off-by: NMarek Vasut <marex@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>
Acked-by: NStefano Babic <sbabic@denx.de>
Acked-by: NStefan Roese <sr@denx.de>