mx6sabresd: Add eMMC specific environment to allow U-Boot update

A new 'update_emmc_firmware' target is added to allow for easy U-Boot
update in the eMMC as it has secury boot partition and this needs
specific handling on how to program the specific partition.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
diff --git a/include/configs/mx6sabresd.h b/include/configs/mx6sabresd.h
index 3229bc7..4919f53 100644
--- a/include/configs/mx6sabresd.h
+++ b/include/configs/mx6sabresd.h
@@ -23,6 +23,8 @@
 #endif
 #define PHYS_SDRAM_SIZE		(1u * 1024 * 1024 * 1024)
 
+#define CONFIG_SUPPORT_EMMC_BOOT /* eMMC specific */
+
 #include "mx6sabre_common.h"
 
 #define CONFIG_SYS_FSL_USDHC_NUM	3