Convert CONFIG_CMD_MMC to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_MMC

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/gurnard_defconfig b/configs/gurnard_defconfig
index a5d51ce..7e394d9 100644
--- a/configs/gurnard_defconfig
+++ b/configs/gurnard_defconfig
@@ -16,6 +16,7 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 # CONFIG_CMD_LOADS is not set
+CONFIG_CMD_MMC=y
 CONFIG_CMD_NAND=y
 CONFIG_CMD_PART=y
 CONFIG_CMD_USB=y