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/configs/bf506f-ezkit_defconfig b/configs/bf506f-ezkit_defconfig
index da25718..09c9899 100644
--- a/configs/bf506f-ezkit_defconfig
+++ b/configs/bf506f-ezkit_defconfig
@@ -1,3 +1,4 @@
 CONFIG_BLACKFIN=y
 CONFIG_TARGET_BF506F_EZKIT=y
 # CONFIG_CMD_BOOTM is not set
+# CONFIG_CMD_EXPORTENV is not set