toradex: apalis/colibri imx6: Select correct DTB for SoM v1.2+

toradex: apalis/colibri imx6: Select correct DTB for SoM v1.2+

When "fdtfile" is not set, use the "variant" environment variable to
select the correct DTB.

Apalis/Colibri iMX6 V1.2 replaced the STMPE811 ADC/Touch controller
which is EOL with the TLA2024 ADC and AD7879 touch controller. They thus
require a different DTB, which we can easily select with the variant env
variable.

Signed-off-by: Ernest Van Hoecke <ernest.vanhoecke@toradex.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>