imx: ventana: config: add PREBOOT support

This allows the 'preboot' env variable to be executed prior to bootcmd
if defined.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

include/configs/gw_ventana.h

@@ -201,6 +201,7 @@
 
 /* Miscellaneous configurable options */
 #define CONFIG_HWCONFIG
+#define CONFIG_PREBOOT
 
 /* Print Buffer Size */
 #define CONFIG_SYS_PBSIZE (CONFIG_SYS_CBSIZE + sizeof(CONFIG_SYS_PROMPT) + 16)