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: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
diff --git a/Kconfig b/Kconfig
index b5f7450..37331d2 100644
--- a/Kconfig
+++ b/Kconfig
@@ -74,6 +74,7 @@
 	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