toradex: colibri_imx7: Enable nand/emmc detection and set boot variant

Add detection of eMMC vs NAND devices on the Colibri iMX7
board. A GPIO is configured to detect the presence of an on-board resistor
that is configured differently based on the flash memory used. Depending on
the detection result, the 'variant' environment variable is set to '-emmc'
or cleared, indicating the type of storage device.

This enhancement improves variant detection during system initialization
through USB recovery mode, where U-Boot is loaded directly to RAM. This
allows variant detection for an accurate device tree selection.

Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>