stubdom/ioemu link farm creation fixes

stubdom/ioemu link farm creation fixes

Replace the stubdom/ioemu link farm creation in stubdom/Makefile,
with code which arranges that:
 * No symlinks are made for output files - in particular, any
   symlinks for .d files would be written through by the compiler
   and cause damage to the original tree and other strange
   behaviours
 * All subdirectories are made as local subdirectories rather than
   links
 * Any interrupted or half-completed creation of the link farm
   leaves the directory in a state where the link farming will be
   restarted
 * We use make's inherent ability to test for the existence of files
   rather than using [ -f ... ] at the start of the rule's commands
 * The list of files to be excluded from the link farm can be
   easily updated
etc.

This should fix some problems particularly with parallel builds,
or by-hand builds where directories are entered in other than the
usual order.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>