buildman: Start the clock when the build starts

The Kconfig and maintainer processing can take a while, sometimes 5
seconds or more. This skews the timing printed by buildmand when the build
completes. Start the clock when the threads start to avoid this problem.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>
diff --git a/tools/buildman/builder.py b/tools/buildman/builder.py
index ecbd368..5305477 100644
--- a/tools/buildman/builder.py
+++ b/tools/buildman/builder.py
@@ -328,7 +328,7 @@
         self._build_period_us = None
         self._complete_delay = None
         self._next_delay_update = datetime.now()
-        self._start_time = datetime.now()
+        self._start_time = None
         self._step = step
         self._error_lines = 0
         self.no_subdirs = no_subdirs
@@ -1778,6 +1778,7 @@
         self._prepare_output_space()
         if not self._ide:
             tprint('\rStarting build...', newline=False)
+        self._start_time = datetime.now()
         self.setup_build(board_selected, commits)
         self.process_result(None)
         self.thread_exceptions = []