buildroot

board/atmel: update documentation for SD card generation

Henceforth, a sdcard.img file is automatically generated. It simplifies the
procedure to generate a bootable SD card for Atmel Xplained boards.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>