buildroot

board/atmel: merge and update documentation

There were two readme files (one for flashing on NAND and one to create a
SD card) in different places. Merge them and update the SD card
generation part since all Xplained boards are supported and the -u
option of sfdisk is obsolete on latest versions.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
[Thomas: adjust the explanations, move the SD card section after the
NAND flash section.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>