uboot-phytec

rk3399: Check MMC env while defining it

rk3399 do support SPI flash as well, so there is
a possibility of using flash environment for those
usecases.

So define env device for MMC only when it is used
by specific configuration.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>