Add Kconfig preprocessor

Implement the Kconfig preprocessor described in
https://github.com/torvalds/linux/blob/master/Documentation/kbuild/kconfig-macro-language.txt
(which is now in linux-next and will appear in Linux 4.18).

A new Kconfig.variables property holds all the preprocessor variables so
that they can be inspected programmatically. Preprocessor variables are
represented by a new Variable class.

With the preprocessor, environment variables are referenced with $(FOO)
instead of $FOO. For backwards compatibility, $FOO is accepted as well
for now (and leaves "$FOO" as-is if FOO doesn't exist). The $FOO syntax
might be dropped at some point in the future (together with a major
version increase). It should be supported for a few months at least.

Some internals were cleaned up too, mostly related to parsing. Some
outdated documentation was fixed as well.

1 files changed, 15 insertions, 3 deletions

diff --git a/README.rst b/README.rst
index 3884e7a..e118834 100644
--- a/README.rst
+++ b/README.rst
@@ -18,6 +18,14 @@ should be relatively easy, if needed.
 The `Zephyr <https://www.zephyrproject.org/>`_ project uses Kconfiglib
 exclusively, with lots of small helper scripts in other projects.
 
+Kconfiglib implements the recently added `Kconfig preprocessor
+<https://github.com/torvalds/linux/blob/master/Documentation/kbuild/kconfig-macro-language.txt>`_.
+For backwards compatibility, environment variables can be referenced both as
+``$(FOO)`` (the new syntax) and as ``$FOO`` (the old syntax). Support for the
+old syntax might be removed in the future (the major version would be increased
+at the same time). Using the old syntax with an undefined environment variable
+keeps the string as is.
+
 Installation
 ------------
 
@@ -193,10 +201,14 @@ The following Kconfig extensions are available:
 - Environment variables are expanded directly in e.g. ``source``
   and ``mainmenu`` statements, meaning ``option env`` symbols
   are redundant.
+
+  This is the standard behavior with the new `Kconfig preprocessor
+  <https://github.com/torvalds/linux/blob/master/Documentation/kbuild/kconfig-macro-language.txt>`_,
+  which Kconfiglib implements.
   
-  ``option env`` symbols are still supported for compatibility,
-  with the caveat that they must have the same name as the environment
-  variables they reference. A warning is printed if the names differ.
+  ``option env`` symbols are supported for backwards compatibility, with the
+  caveat that they must have the same name as the environment variables they
+  reference. A warning is printed if the names differ.
 
 Other features
 --------------