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/bcm7260_defconfig b/configs/bcm7260_defconfig
index afb59ec..d467e62 100644
--- a/configs/bcm7260_defconfig
+++ b/configs/bcm7260_defconfig
@@ -13,6 +13,7 @@
 CONFIG_PREBOOT="fdt addr ${fdtcontroladdr};fdt move ${fdtcontroladdr} ${fdtsaveaddr};fdt addr ${fdtsaveaddr};"
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot>"
+CONFIG_CMD_MMC=y
 CONFIG_EFI_PARTITION=y
 CONFIG_OF_PRIOR_STAGE=y
 CONFIG_ENV_IS_IN_MMC=y