This is copied from U-Boot v2014.10 and modified for use with
barebox:

- Add a main() function to make it a standalone tool
- Add option to pass in the prepare stage and bootloader
  image as options. If the config file contains @PREP@ or
  @BOOTLOADER@ the string will be replaced with the actual
  image file passed via the -p and -b options.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>