Config.in: Show BR version in title

Kconfig nowadays uses the mainmenu as the title, so add the version info
here, similar to how it is done in the Linux kernel.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Config.in

@@ -1,6 +1,6 @@
 #
 
-mainmenu "Buildroot2 Configuration"
+mainmenu "Buildroot $BR2_VERSION Configuration"
 
 config BR2_HAVE_DOT_CONFIG
 	bool