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/README b/tools/buildman/README
index b8c2bd6..fbc8449 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -442,7 +442,8 @@
 we added some code and moved the broken line father down the file.
 
 If many boards have the same error, then -e will display the error only
-once. This makes the output as concise as possible.
+once. This makes the output as concise as possible. To see which boards have
+each error, use -l.
 
 The full build output in this case is available in:
 
@@ -745,10 +746,10 @@
 to see the results of the build. Rather than showing you all the output,
 buildman just shows a summary, with red indicating that a commit introduced
 an error and green indicating that a commit fixed an error. Use the -e
-flag to see the full errors.
+flag to see the full errors and -l to see which boards caused which errors.
 
 If you really want to see build results as they happen, use -v when doing a
-build (and -e if you want to see errors as well).
+build (-e will be enabled automatically).
 
 You don't need to stick around on that branch while buildman is running. It
 checks out its own copy of the source code, so you can change branches,