restore documentation on the build config language

Kconfig-language.txt was deleted in commit 4fa499a17b back in 2006.
Move to docs/ as suggested by Xabier Oneca:
  http://lists.busybox.net/pipermail/busybox/2014-May/080914.html
Also update references to it everywhere.

Signed-off-by: Kartik Agaram <akkartik@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

klibc-utils/Config.src

@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see scripts/kbuild/config-language.txt.
+# see docs/Kconfig-language.txt.
 #
 
 menu "klibc-utils"