configs: colibri-imx7*: set 'fdtfile' generically in PREBOOT

configs: colibri-imx7*: set 'fdtfile' generically in PREBOOT

In TEZI (Toradex Easy Installer), we use one U-Boot binary for both our
NAND and eMMC Colibri iMX7 modules. Currently, CONFIG_PREBOOT sets the
environment variable 'fdtfile' depending on which defconfig was used,
adding the '-emmc' variant for the emmc defconfig. Since we always build
the TEZI recovery U-Boot with the standard (non-emmc) defconfig, fdtfile
has to be overwritten later or it will be wrong there.

By using '$variant', the fdtfile var is properly constructed at run time
for both the NAND and eMMC variants, and we do not have to worry about
setting fdtfile again when building the recovery TEZI U-Boot.

This also synchronizes these configs with how we handle the iMX6ULL.

Fixes: 327381e8b57c ("colibri_imx7: use preboot for fdtfile evaluation")
Signed-off-by: Ernest Van Hoecke <ernest.vanhoecke@toradex.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>