mx6ul_evk: Remove CONFIG_SUPPORT_EMMC_BOOT

mx6ul_evk does not come with a eMMC populated, so we should not
define CONFIG_SUPPORT_EMMC_BOOT as it causes SPL to not be able
to boot some brands of SD cards, such as SanDisk microSD HC - 8GB:

U-Boot SPL 2016.05-rc1-28384-g108f841 (Apr 19 2016 - 11:19:11)
Trying to boot from MMC1
spl: mmc block read error
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

When CONFIG_SUPPORT_EMMC_BOOT is defined spl_boot_mode() returns
MMCSD_MODE_EMMCBOOT, so remove this option to have a reliable boot
via SD card.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>