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/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index 66541c0..1f0b05b 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -52,6 +52,8 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_SYS_MMC_ENV_DEV=1
+CONFIG_SYS_MMC_ENV_PART=2
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_BOOTCOUNT_LIMIT=y