Overwrite ${board}_file in local-remote-host

After I run test like this,

 $ make check RUNTESTFLAGS='--host_board=local-remote-host dw2-basic.exp'

gdb.dwarf2/file1.txt in source tree was removed.  In some gdb.dwarf2/*.exp,
file1.txt is copied to host and then removed at the end.  However, in
local-remote-host-notty.exp, ${board}_download doesn't copy the file but
return the absolute path of the src file.  'remote_file host delete' at
the end will remove the file in source tree.

This patch is to overwrite ${board}_file, and specially make "delete"
option do nothing.  This approach is used in gdbserver-base.exp and
remote-stdio-gdbserver.exp too.

gdb/testsuite:

2014-05-14  Yao Qi  <yao@codesourcery.com>

	* boards/local-remote-host-notty.exp (${board}_file): New
	proc.