buildman: Add a way to build a particular target

At present buildman only supports building the default target. Generally
this is what is wanted, but in some cases boards erroneously have a
different target for product extra files.

Add a --target option to help. Also add a comment indicating which
letters are free for new options.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builder.py b/tools/buildman/builder.py
index 4bea0a0..6538a3d 100644
--- a/tools/buildman/builder.py
+++ b/tools/buildman/builder.py
@@ -265,7 +265,7 @@
                  reproducible_builds=False, force_build=False,
                  force_build_failures=False, force_reconfig=False,
                  in_tree=False, force_config_on_failure=False, make_func=None,
-                 dtc_skip=False):
+                 dtc_skip=False, build_target=None):
         """Create a new Builder object
 
         Args:
@@ -315,6 +315,7 @@
                 retrying a failed build
             make_func (function): Function to call to run 'make'
             dtc_skip (bool): True to skip building dtc and use the system one
+            build_target (str): Build target to use (None to use the default)
         """
         self.toolchains = toolchains
         self.base_dir = base_dir
@@ -363,6 +364,7 @@
                 raise ValueError('Cannot find dtc')
         else:
             self.dtc = None
+        self.build_target = build_target
 
         if not self.squash_config_y:
             self.config_filenames += EXTRA_CONFIG_FILENAMES