boot/uboot: add support for i.MX28 SD format

Allow to generate u-boot.sd, Freescale i.MX28 SB format with a header for
booting from an SD card.

U-boot includes an mxsboot tool to generate this format, starting from
v2011.12.

See u-boot doc/README.mxs (or doc/README.mx28_common before 2013.07).

[Arnout:
 - Wrap Config.in help text at 72 columns.
 - Remove v from U-Boot version specifier.
 - Refer to different README name before 2013.07.
 - Depend on host-elftosb
 - Add comment to clarify sb -> sd conversion.]

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Tested-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>