arm: mvebu: ds414: Add sample u-boot update command

Call 'run update_uboot' to fetch u-boot-spl.kwb via TFTP and write it
into the correct SPI flash location. The latter's size is defined in
DS414's DTB file, so hard-coding it should be acceptable here.

Take care to not append garbage from RAM to the written image and to
stay within assigned flash boundaries even if an oversized image was
fetched.

Signed-off-by: Phil Sutter <phil@nwl.cc>