Added a new attribute IMPORT to the CONDES definition of the FEATURES section.

Added a new attribute IMPORT to the CONDES definition of the FEATURES section.
If this attribute is defined, an import for the given symbol is added to the
module that contains the condes declaraction. Using this feature, it is
possible to force linkage of a module that contains an export for the symbol.

git-svn-id: svn://svn.cc65.org/cc65/trunk@5900 b7a2c559-68d2-44c3-8de9-860c34a00d81