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

Cc: Priyanka Jain <priyanka.jain@nxp.com>
Cc: Madalin Bucur <madalin.bucur@oss.nxp.com>
Cc: Hannes Schmelzer <hannes.schmelzer@br-automation.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
---
Due to some non-environment usage of the variables, cc'ing a few folks.

Changes in v2:
- A few more migrations