mx6cuboxi: Use mmc_get_op_cond() to check for an eMMC

Previously we had just made broad assumptions with which of our
boards had an eMMC or not even though this is a manufacturing time
assembly option.  This takes the guessing away and actually checks for
the existence of an eMMC and sets up the has_emmc environment variable.

Signed-off-by: Jon Nettleton <jon@solid-run.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>