cros_bundle_firmware: Move firmware writing into write_firmware.py

This function should not be in cros_bundle_firmware - we move it into the
correct place so that it can be used with cros_write_firmware also.

BUG=chromium-os:17298
TEST=run cros_bundle_firmware -w and see that it still works

Change-Id: If1ab0c7ad1bc9f7bc424ba072c574a8ca8df1356
Reviewed-on: http://gerrit.chromium.org/gerrit/5591
Reviewed-by: Che-Liang Chiou <clchiou@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/host/lib/write_firmware.py b/host/lib/write_firmware.py
index 00cd17f..0478b26 100644
--- a/host/lib/write_firmware.py
+++ b/host/lib/write_firmware.py
@@ -205,3 +205,24 @@
         time.sleep(1)
 
     return False
+
+def DoWriteFirmware(output, tools, fdt, flasher, bct_fname, image_fname):
+  """A simple function to write firmware to the board.
+
+  This creates a WriteFirmware object and uses it to write the firmware image
+  to the board.
+
+  Args:
+    output: cros_output object to use.
+    tools: Tools object to use.
+    fdt: Fdt object to use as our device tree.
+    flasher: U-Boot binary to use as the flasher.
+    bct_fname: Bct file to use for the flasher.
+    image_fname: Filename of image to write.
+  """
+  write = WriteFirmware(tools, fdt, output)
+  if write.FlashImage(flasher, bct_fname, image_fname):
+    output.Progress('Image uploaded - please wait for flashing to '
+        'complete')
+  else:
+    raise CmdError('Image upload failed - please check board connection')