distro: select CMD_ENV_EXISTS

'env exists' is used for scripting in include/config_distro_bootcmd.h
This command is available only when CONFIG_CMD_ENV_EXISTS is enabled.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NAlexander Graf <agraf@suse.de>

Kconfig

@@ -74,6 +74,7 @@ config DISTRO_DEFAULTS
 	select CMD_DHCP if CMD_NET
 	select CMD_PING if CMD_NET
 	select CMD_PXE if NET
+	select CMD_ENV_EXISTS
 	select CMD_EXT2
 	select CMD_EXT4
 	select CMD_FAT