1. Update conditional markup:

1. Update conditional markup:
 (a) to make H8 configuration come out right (first attempt since
     conversion to texinfo conditionals);
 (b) to exploit yesterday's makeinfo bugfix, using conditionals in menus
     rather than multiple alternative menus.

2. Update H8 text to refer to H8/500 as well as H8/300.

3. Define config file gdb-config.texi as a link in Makefile, based on a
   DOC_CONFIG variable.