cmd: Add a SEAMA image load command

Add a command to load SEAMA (Seattle Image), a NAND flash
on-flash storage format.

This type of flash image is found in some D-Link routers such
as DIR-645, DIR-842, DIR-859, DIR-860L, DIR-885L, DIR890L and
DCH-M225, as well as in WD and NEC routers on the ath79
(MIPS), Broadcom BCM53xx, and RAMIPS platforms.

This U-Boot command will read and decode a SEAMA image from
raw NAND flash on any platform. As it is always using big endian
format for the data decoding is always necessary on platforms
such as ARM.

The command is needed to read a SEAMA-encoded boot image on the
D-Link DIR-890L router for boot from NAND flash in an upcoming
port of U-Boot to the Broadcom Northstar (BCM4709, BCM53xx)
architecture.

A basic test and documentation is added as well. The test must
be run on a target with NAND flash support and at least one
resident SEAMA image in flash.

Cc: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>