configs: rename atmel_xplained_defconfig

For consistency, rename atmel_xplained_defconfig to
atmel_sama5d3_xplained_mmc_defconfig as this configuration supports
booting the sama5d3 Xplained with an MMC card (as opposed to the NAND
flash).

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/atmel/xplained/readme.txt

@@ -10,19 +10,19 @@ web site.
 Note that the default Buildroot configuration is prepared to boot from
 an SD card: the first stage bootloader, second stage bootloader,
 kernel image and root filesystem are all located on the SD card. Some
-adjustements in the bootloader configuration will be needed to boot
-from NAND flash.
+adjustments in the bootloader configuration will be needed to boot
+from NAND flash, see board/atmel/nandflash.txt
 
 Configuring and building Buildroot
 ----------------------------------
 
-  make atmel_xplained_defconfig
+  make atmel_sama5d3_xplained_mmc_defconfig
   make
 
 Preparing the SD card
 ---------------------
 
-The SD card must be partitionned with at least two partitions: one
+The SD card must be partitioned with at least two partitions: one
 FAT16 partition for the bootloaders, kernel image and Device Tree
 blob, and one ext4 partition for the root filesystem. To partition the
 SD card: