kbuild: document the KBUILD_KCONFIG env. variable

Add usage info for the Kbuild environment variable KBUILD_KCONFIG.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/kbuild.txt

@@ -50,6 +50,11 @@ LDFLAGS_MODULE
 --------------------------------------------------
 Additional options used for $(LD) when linking modules.
 
+KBUILD_KCONFIG
+--------------------------------------------------
+Set the top-level Kconfig file to the value of this environment
+variable.  The default name is "Kconfig".
+
 KBUILD_VERBOSE
 --------------------------------------------------
 Set the kbuild verbosity. Can be assigned same values as "V=...".