[gdb/testsuite] Fix gdb.base/break-probes.exp with native-gdbserver

[gdb/testsuite] Fix gdb.base/break-probes.exp with native-gdbserver

When running break-probes.exp with native-gdbserver, we run into:
...
FAIL: gdb.base/break-probes.exp: run til our library loads (the program exited)
FAIL: gdb.base/break-probes.exp: call (int) foo(23)
...
due to the fact that we're trying to match:
...
Inferior loaded /data/gdb_versions/devel/build/gdb/testsuite/outputs/gdb.base\
  /break-probes/break-probes-solib.so
...
using pattern:
...
Inferior loaded $sysroot$binfile_lib
...
which expands into:
...
Inferior loaded //data/gdb_versions/devel/build/gdb/testsuite/outputs/gdb.base\
  /break-probes/break-probes-solib.so
...

Fix by setting sysroot to "" in local-board.exp.

Tested on x86_64-linux with native-gdbserver.

gdb/testsuite/ChangeLog:

2019-04-18  Tom de Vries  <tdevries@suse.de>

	PR gdb/24433
	* boards/local-board.exp: Set sysroot to "".