Separate processing the linker config file into two phases: The config file is

Separate processing the linker config file into two phases: The config file is
read when the -t or -C switch is encountered and parts of it are processed.
The remaining parts are processed when all object files and libraries have
been read. To make this work, the expression evaluation in cfgexpr has been
rewritten to generate true expression trees. This means that expressions in
the linker config may use exports from the object files.

Separation of config file processing is the base for several enhancements, for
example forced imports by linker config.

This code needs more work and is only very, very, very roughly tested.

git-svn-id: svn://svn.cc65.org/cc65/trunk@4840 b7a2c559-68d2-44c3-8de9-860c34a00d81