config_cmd_default.h: Add 'env exists' command

env exists allows scripts to query whether an environment variable
exists. Enable by default as it adds only a trivial amount of code and
can be useful in scripts.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>

include/config_cmd_default.h

@@ -21,6 +21,7 @@
 #define CONFIG_CMD_CONSOLE	/* coninfo			*/
 #define CONFIG_CMD_ECHO		/* echo arguments		*/
 #define CONFIG_CMD_EDITENV	/* editenv			*/
+#define CONFIG_CMD_ENV_EXISTS	/* query whether env variables exists */
 #define CONFIG_CMD_FPGA		/* FPGA configuration Support	*/
 #define CONFIG_CMD_IMI		/* iminfo			*/
 #define CONFIG_CMD_ITEST	/* Integer (and string) test	*/