scons: add ParseSourceList method

scons: add ParseSourceList method

ParseSourceList() can be used to parse a source list file and returns
the source files defined in it.  It is supposed to be used like this

  # get the list of source files from C_SOURCES in Makefile.sources
  sources = env.ParseSourceList('Makefile.sources', 'C_SOURCES')

The syntax of a source list file is compatible with GNU Make.  This
effectively allows SConscript and Makefile to share the source lists.

Acked-by: José Fonseca <jfonseca@vmware.com>
Acked-by: Chad Versace <chad@chad-versace.us>