buildman: Add an option to just create the config

Normally buildman does a full build of a board. This includes creating the
u-boot.cfg file which contains all the configuration options. Buildman uses
this file with the -K option, to show differences in effective configuration
for each commit.

Doing a full build of U-Boot just to create the u-boot.cfg file is wasteful.
Add a -D option which causes buildman to only create the configuration. This
is enough to support use of -K and can be done much more quickly (typically
5-10 times faster).

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builder.py b/tools/buildman/builder.py
index e27a285..d834d31 100644
--- a/tools/buildman/builder.py
+++ b/tools/buildman/builder.py
@@ -207,7 +207,8 @@
     def __init__(self, toolchains, base_dir, git_dir, num_threads, num_jobs,
                  gnu_make='make', checkout=True, show_unknown=True, step=1,
                  no_subdirs=False, full_path=False, verbose_build=False,
-                 incremental=False, per_board_out_dir=False):
+                 incremental=False, per_board_out_dir=False,
+                 config_only=False):
         """Create a new Builder object
 
         Args:
@@ -230,6 +231,7 @@
                 mrproper when configuring
             per_board_out_dir: Build in a separate persistent directory per
                 board rather than a thread-specific directory
+            config_only: Only configure each build, don't build it
         """
         self.toolchains = toolchains
         self.base_dir = base_dir
@@ -257,6 +259,7 @@
         self.no_subdirs = no_subdirs
         self.full_path = full_path
         self.verbose_build = verbose_build
+        self.config_only = config_only
 
         self.col = terminal.Color()