arm/km: add CONFIG_KM_BOARD_EXTRA_ENV to default environment

This is needed for board specific additional environment variables.
E.g. the mgcoge3un has this additional "waitforne" variable.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Acked-by: Valentin Longchamp <valentin.longchamp@keymile.com>

include/configs/km/km_arm.h

@@ -289,10 +289,15 @@ int get_scl(void);
 		" ${addr} " __stringify(CONFIG_ENV_OFFSET_REDUND) " 4\0"
 #endif
 
+#ifndef CONFIG_KM_BOARD_EXTRA_ENV
+#define CONFIG_KM_BOARD_EXTRA_ENV       ""
+#endif
+
 /*
  * Default environment variables
  */
 #define CONFIG_EXTRA_ENV_SETTINGS					\
+	CONFIG_KM_BOARD_EXTRA_ENV					\
 	CONFIG_KM_DEF_ENV						\
 	CONFIG_KM_NEW_ENV						\
 	"arch=arm\0"							\