kconfig: move CONFIG_CMD_EXPORTENV to Kconfig

Since CONFIG_CMD_EXPORTENV 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>
diff --git a/common/Kconfig b/common/Kconfig
index 5dfac25..f628315 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -13,4 +13,10 @@
 	help
 	  Compute CRC32.
 
+config CMD_EXPORTENV
+	bool "Enable env export command"
+	default y
+	help
+	  Export environments.
+
 endmenu