[XEN][BUILD] Both ctags and etags support the -a (append) option, and

[XEN][BUILD] Both ctags and etags support the -a (append) option, and
by removing the tags or TAGS file, and then running using the
-a option in conjunction with xargs a full list of tags will
be obtained, regardless of how many files there are.

I believe that the existing invocations of both etags and ctags are wrong:

* I don't think - is a vailid argument to etags

* xargs ctags does not handle the case where ctags is invoked
  multiple times by xargs when there are too many files
  to fit on one command line.

Signed-off-by: Horms <horms@verge.net.au>