kconfig: add built-in function support

This commit adds a new concept 'function' to do more text processing
in Kconfig.

A function call looks like this:

  $(function,arg1,arg2,arg3,...)

This commit adds the basic infrastructure to expand functions.
Change the text expansion helpers to take arguments.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>