Reworked and improved the SYMBOLS section. The old syntax (using symbol =

Reworked and improved the SYMBOLS section. The old syntax (using symbol =
value) is now gone, attributes are used instead. The SYMBOLS section does now
support imports, so the linker config can be used to force symbols (and
therefore module) imports. Evaluation of start address and size for memory
areas has been delayed even further, so it is now possible to use the values
from one memory area in the definition of the next one.

git-svn-id: svn://svn.cc65.org/cc65/trunk@4851 b7a2c559-68d2-44c3-8de9-860c34a00d81