config: Update environment variable usage

Upstream changes in the way kconfig handles environment variables means
we need to update our usage of them.

Signed-off-by: Chris Packham <judge.packham@gmail.com>

config/config.in

@@ -1,4 +1,4 @@
-mainmenu "crosstool-NG $CT_VERSION Configuration"
+mainmenu "crosstool-NG $(CT_VERSION) Configuration"
 source "config/configure.in"
 source "config/global.in"
 source "config/target.in"

config/global.in

@@ -2,7 +2,7 @@
 
 config VERSION
     string
-    option env="CT_VERSION"
+    default "$(CT_VERSION)"
 
 # Config version checking framework. If CONFIG_VERSION is unset in the current .config
 # or defconfig, it is loaded as 0 (possibly triggering a user prompt during 'ct-ng oldconfig').
@@ -16,13 +16,13 @@ config VERSION
 # i.e. 'ct-ng saveconfig' or 'ct-ng savedefconfig'.
 config VCHECK
     string
-    option env="CT_VCHECK"
+    default "$(CT_VCHECK)"
 
 # Kconfig's idiosynchrasy: it does not output the config settings declared with 'option env'
 # to the generated .config file. Must use one extra level of indirection.
 config CONFIG_VERSION_ENV
     string
-    option env="CT_CONFIG_VERSION_CURRENT"
+    default "$(CT_CONFIG_VERSION_CURRENT)"
 
 # Up-to-date version of the configuration file. If saving a defconfig, choose a default that
 # is guaranteed not to match - so that kconfig saves the value to .config.