buildroot

uboot: mark Freescale i.MX28 specific formats as such

It's rather unclear that uboot.sb, uboot.sd and uboot.nand are
Freescale i.MX28 specific, so let's make that clear in the prompt of
each option, like we do for the Marvell-specific uboot.kwb.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>