Kconfig: Use new preprocessor syntax for env. variables

With the new Kconfig preprocessor (described in
https://github.com/torvalds/linux/blob/master/Documentation/kbuild/


kconfig-macro-language.txt), the syntax for expanding environment
variables is $(FOO) rather than $FOO.

$(FOO) is a general preprocessor variable expansion, which falls back to
environment variables if the variable isn't set (like in Make). It can
also be used in prompts, 'comment's, etc.

The old syntax will probably be supported forever in Kconfiglib for
backwards compatibility, but might as well make it consistent now that
people might start using the preprocessor more.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>