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/ls1021aiot_sdcard_defconfig b/configs/ls1021aiot_sdcard_defconfig
index fc6df70..10b6930 100644
--- a/configs/ls1021aiot_sdcard_defconfig
+++ b/configs/ls1021aiot_sdcard_defconfig
@@ -17,6 +17,7 @@
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0xe8
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GPT=y
+CONFIG_CMD_MMC=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_EXT2=y