buildman: Add a flag to force mrproper on failure

When a file is removed by a commit (e.g. include/common.h yay!) it can
cause incremental build failures since one of the dependency files from
a previous build may mention the file.

Add an option to run 'make mrproper' automatically when a build fails.
This can be used to automatically resolve the problem, without always
adding the large overhead of 'make mrproper' to every build.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/buildman.rst b/tools/buildman/buildman.rst
index aae2477..bd0482a 100644
--- a/tools/buildman/buildman.rst
+++ b/tools/buildman/buildman.rst
@@ -995,7 +995,8 @@
 first commit for each board. This reduces the amount of work 'make' does, and
 hence speeds up the build. To force use of 'make mrproper', use -the -m flag.
 This flag will slow down any buildman invocation, since it increases the amount
-of work done on any build.
+of work done on any build. An alternative is to use the --fallback-mrproper
+flag, which retries the build with 'make mrproper' only after a build failure.
 
 One possible application of buildman is as part of a continual edit, build,
 edit, build, ... cycle; repeatedly applying buildman to the same change or