arm: imx: Pack u-boot.bin into uImage for SPL

The U-Boot SPL can parse the uImage format and gather information from
it about the payload. Make use of this and wrap u-boot.bin into uImage
format. The benefit is the SPL will know the size of the payload
instead of using fixed size of the payload defined at compile time.

Signed-off-by: Marek 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>