buildman: Add an option to show which boards caused which errors

Add a -l option to display a list of offending boards against each
error/warning line. The information will be shown in brackets as below:

02: wip
   sandbox: +   sandbox
       arm: +   seaboard
+(sandbox) arch/sandbox/cpu/cpu.c: In function 'timer_get_us':
+(sandbox) arch/sandbox/cpu/cpu.c:40:9: warning: unused variable 'i' [-Wunused-variable]
+(seaboard) board/nvidia/seaboard/seaboard.c: In function 'pin_mux_mmc':
+(seaboard) board/nvidia/seaboard/seaboard.c:36:9: warning: unused variable 'fred' [-Wunused-variable]
+(seaboard)      int fred;
+(seaboard)          ^

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/buildman.py b/tools/buildman/buildman.py
index 53592e5..1258b76 100755
--- a/tools/buildman/buildman.py
+++ b/tools/buildman/buildman.py
@@ -94,6 +94,8 @@
        default=None, help='Number of jobs to run at once (passed to make)')
 parser.add_option('-k', '--keep-outputs', action='store_true',
        default=False, help='Keep all build output files (e.g. binaries)')
+parser.add_option('-l', '--list-error-boards', action='store_true',
+       default=False, help='Show a list of boards next to each error/warning')
 parser.add_option('--list-tool-chains', action='store_true', default=False,
        help='List available tool chains')
 parser.add_option('-n', '--dry-run', action='store_true', dest='dry_run',