Convert CONFIG_ENV_IS_IN_SPI_FLASH et al to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH
   CONFIG_ENV_IS_NOWHERE
   CONFIG_ENV_OFFSET
   CONFIG_ENV_OVERWRITE
   CONFIG_ENV_SECT_SIZE
   CONFIG_SYS_MMC_ENV_DEV
   CONFIG_SYS_MMC_ENV_PART

Signed-off-by: Tom Rini <trini@konsulko.com>