kconfig: move CONFIG_CMD_IMPORTENV to Kconfig

Since CONFIG_CMD_IMPORTENV is defined in config_cmd_defaults.h,
it should be enabled for all the boards except bf506f-ezkit
that undefs it explicitely.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>

common/Kconfig

@@ -19,6 +19,12 @@ config CMD_EXPORTENV
 	help
 	  Export environments.
 
+config CMD_IMPORTENV
+	bool "Enable env import command"
+	default y
+	help
+	  Import environments.
+
 config CMD_GO
 	bool "Enable go command"
 	default y