Convert CONFIG_SYS_MMC_ENV_DEV et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MMC_ENV_DEV
   CONFIG_SYS_MMC_ENV_PART

Note that with this conversion we now have consistent behavior with
respect to ensuring that we have always selected the correct MMC
device and hardware partition.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/riotboard_defconfig b/configs/riotboard_defconfig
index c33ea6e..1aeeb08 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -23,6 +23,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_SYS_MMC_ENV_DEV=2
 CONFIG_DM=y
 CONFIG_SUPPORT_EMMC_BOOT=y
 CONFIG_FSL_USDHC=y