These changes tighten up rules for exporting options.

These changes tighten up rules for exporting options.

1. Exportable options ('export used') used to be exported if referred to in
   a 'uses' statement. These options will now only be exported if the
   option is set, or the default value is changed.

2. Options marked as 'export always' with no default value ('default none')
   used to generate defines with no values 'export k8:='. This behavior
   has changed so that the option will ONLY be exported if it has a value
   assigned using 'set' or 'default'. Otherwise it is an error.

git-svn-id: svn://svn.coreboot.org/coreboot/trunk@1467 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1