testsuite: Add option to capture gdbserver debug

testsuite: Add option to capture gdbserver debug

Add both board option and environment variable which enables gdbserver
debug and sends it to the file gdbserver.debug, located in the output
directory for the current test.  Document this.

Add support for the environment variable in the Makefile.

The testsuite can be run with gdbserver debug enabled in the following way:

	make check GDBSERVER_DEBUG=all

Disable tspeed.exp when debugging to prevent the log file filling
many gigabytes then timing out.

gdb/testsuite/ChangeLog:

	* Makefile.in: Pass through GDBSERVER_DEBUG.
        * README (Testsuite Parameters): Add GDBSERVER_DEBUG.
        (gdbserver,debug): Add board setting.
        * gdb.trace/tspeed.exp: Skip when debugging.
	* lib/gdb.exp (gdbserver_debug_enabled): New procedure.
	* lib/gdbserver-support.exp: Likewise