buildman: Make -I the default

At present buildman defaults to running 'mrproper' on every thread before
it starts building commits for each board. This can add a delay of about 5
seconds to the start of the process, since the tools and other invariants
must be rebuilt.

In particular, a build without '-b', to build current source, runs much
slower without -I, since any existing build is removed, thus losing the
possibility of an incremental build.

Partly this behaviour was to avoid strange build-system problems caused by
running 'make defconfig' for one board and then one with a different
architecture. But these problems were fixed quite a while ago.

The -I option (which disabled mrproper) was introduced four years ago and
does not seem to cause any problems with builds.

So make -I the default and deprecate the option. To allow use of
'mrproper', add a new -m flag.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/cmdline.py b/tools/buildman/cmdline.py
index 17ea015..0178c6e 100644
--- a/tools/buildman/cmdline.py
+++ b/tools/buildman/cmdline.py
@@ -55,8 +55,9 @@
     parser.add_option('-i', '--in-tree', dest='in_tree',
           action='store_true', default=False,
           help='Build in the source tree instead of a separate directory')
+    # -I will be removed after April 2021
     parser.add_option('-I', '--incremental', action='store_true',
-          default=False, help='Do not run make mrproper (when reconfiguring)')
+          default=False, help='Deprecated, does nothing. See -m')
     parser.add_option('-j', '--jobs', dest='jobs', type='int',
           default=None, help='Number of jobs to run at once (passed to make)')
     parser.add_option('-k', '--keep-outputs', action='store_true',
@@ -69,6 +70,8 @@
           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 (use -v to see probing detail)')
+    parser.add_option('-m', '--mrproper', action='store_true',
+          default=False, help="Run 'make mrproper before reconfiguring")
     parser.add_option('-n', '--dry-run', action='store_true', dest='dry_run',
           default=False, help="Do a dry run (describe actions, but do nothing)")
     parser.add_option('-N', '--no-subdirs', action='store_true', dest='no_subdirs',