[iMX6][SPL] Improve definition of CONFIG_SUPPORT_EMMC_BOOT to select boot mode

The definition of `CONFIG_SUPPORT_EMMC_BOOT` in `mx6_common.h` has been
updated adding the value 1 to it. Mostly, this config is processed
only checking if it's defined and for these cases the former
definition was enough.

For iMX6, in `arch/arm/mach-imx/spl.c`, the config is checked with
`IS_ENABLED()` and it needs to have a value in order to pass the check.

This caused the boot mode always to `MMCSD_MODE_RAW` with the former
definition.

Signed-off-by: Nicola Sparnacci <nicola.sparnacci@seco.com>

include/configs/mx6_common.h

@@ -60,7 +60,7 @@
 #define PHY_ANEG_TIMEOUT 20000
 
 /* MMC */
-#define CONFIG_SUPPORT_EMMC_BOOT
+#define CONFIG_SUPPORT_EMMC_BOOT 1
 
 #ifdef CONFIG_SPL_BUILD
 #define CONFIG_SPL_DRIVERS_MISC_SUPPORT

include/configs/seco_imx6dl_santino.h

@@ -3,13 +3,6 @@
 
 #include "seco_mx6_common.h"
 
-
-// Override CONFIG_SUPPORT_EMMC_BOOT defined in mx6_common.h to enable booting 
-// from mmcblkXbootY partitions. In the file mx6_common.h the macro is defined
-// without any value so that IS_ENABLED() macros in arch/arm/mach-imx/spl.c 
-// erroneously fail.
-#define CONFIG_SUPPORT_EMMC_BOOT 1
-
 /* ____________________________________________________________________________
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