ARM: at91: at91sam9rlek: add mmc environment configuration

Add a mmc default config, which will save the environment in a FAT file
(uboot.env) of MMC.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
diff --git a/configs/at91sam9rlek_mmc_defconfig b/configs/at91sam9rlek_mmc_defconfig
new file mode 100644
index 0000000..b93c881
--- /dev/null
+++ b/configs/at91sam9rlek_mmc_defconfig
@@ -0,0 +1,3 @@
+CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9RL,SYS_USE_MMC"
+CONFIG_ARM=y
+CONFIG_TARGET_AT91SAM9RLEK=y