We now have second stage support for running from sdmmc0 and sdmmc1.
Add a barebox environment and update handler for the two SD cards.

As fall back, we use the environment in the QSPI flash as before as this
is soldered to the SoM and is always available.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>