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-g108f8418 (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: NFabio Estevam <fabio.estevam@nxp.com>

include/configs/mx6ul_14x14_evk.h

@@ -45,7 +45,6 @@
 #define CONFIG_SYS_FSL_USDHC_NUM	2
 #endif
 
-#define CONFIG_SUPPORT_EMMC_BOOT /* eMMC specific */
 #endif
 
 /* I2C configs */