Set timeout for gdb.reverse/*.exp test cases

Set timeout for gdb.reverse/*.exp test cases

Hi,
This patch is to add a new board setting gdb_reverse_timeout, which is
used to set timeout for all gdb.reverse test cases, which are usually
very slow and cause some TIMEOUT failures, for example, on some arm
boards.  We have some alternatives to this approach, but I am not
satisfied with them:

 - Increase the timeout value.  This is the global change, and it may
   cause some delay where actual failures happen.
 - Set timeout by gdb_reverse_timeout in every gdb.reverse/*.exp.
   Then, we have to touch every file under gdb.reverse.

In this patch, we choose a central place to set timeout for all tests
in gdb.reverse, which is convenient.

gdb/testsuite:

2014-05-20  Yao Qi  <yao@codesourcery.com>

	* lib/gdb.exp (gdb_init): Set timeout if test file is under
	gdb.reverse directory and gdb_reverse_timeout exists in board
	setting.
	* README: Document gdb_reverse_timeout.