buildman: Allow ignoring warnings in the return code

Sometimes we don't want buildman to return failure if it seems warnings.
Add a -W option to support this. If buildman detects warnings (and no
errors) it will return an exit code of 0 (success).

Note that the definition of 'warnings' includes the migration warnings
produced by U-Boot, such as:

    ===================== WARNING ======================
    This board does not use CONFIG_DM_MMC. Please update
    ...
    ====================================================

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

tools/buildman/README

@@ -1070,16 +1070,32 @@ This will write the full build into /tmp/build including object files.
 Other options
 =============
 
-Buildman has various other command line options. Try --help to see them.
+Buildman has various other command-line options. Try --help to see them.
 
 To find out what architecture or toolchain prefix buildman will use for a build,
 see the -a and -A options.
 
+To request that compiler warnings be promoted to errors, use -E. This passes the
+-Werror flag to the compiler. Note that the build can still produce warnings
+with -E, e.g. the migration warnings:
+
+        ===================== WARNING ======================
+        This board does not use CONFIG_DM_MMC. Please update
+        ...
+        ====================================================
+
 When doing builds, Buildman's return code will reflect the overall result:
 
     0 (success)     No errors or warnings found
     128             Errors found
-    129             Warnings found
+    129             Warnings found (only if no -W)
+
+You can use -W to tell Buildman to return 0 (success) instead of 129 when
+warnings are found. Note that it can be useful to combine -E and -W. This means
+that all compiler warnings will produce failures (code 128) and all other
+warnings will produce success (since 129 is changed to 0).
+
+If there are both warnings and errors, errors win, so buildman returns 128.
 
 
 How to change from MAKEALL

tools/buildman/cmdline.py

@@ -108,6 +108,8 @@ def ParseArgs():
           default=False, help='Run make with V=1, logging all output')
     parser.add_option('-w', '--work-in-output', action='store_true',
           default=False, help='Use the output directory as the work directory')
+    parser.add_option('-W', '--ignore-warnings', action='store_true',
+          default=False, help='Return success even if there are warnings')
     parser.add_option('-x', '--exclude', dest='exclude',
           type='string', action='append',
           help='Specify a list of boards to exclude, separated by comma')

tools/buildman/control.py

@@ -386,6 +386,6 @@ def DoBuildman(options, args, toolchains=None, make_func=None, boards=None,
                                 options.keep_outputs, options.verbose)
             if fail:
                 return 128
-            elif warned:
+            elif warned and not options.ignore_warnings:
                 return 129
     return 0