As almost all sama5 sd/mmc env configurations are same, so move them to
at91-sama5_common.h.

Also define CONFIG_ENV_VARS_UBOOT_CONFIG to have the varaible: $board_name.
Then we can use 'at91-${board_name}.dtb' as the dtb name.

TODO: since sama5d3xek has different dtb name, we need to some extra
stuff to make it work on sama5d3xek boards.
Signed-off-by: NJosh Wu <josh.wu@atmel.com>