Add gzipped logo support

The README file states that CONFIG_VIDEO_BMP_GZIP behaves as follows:

  If this option is set, additionally to standard BMP
  images, gzipped BMP images can be displayed via the
  splashscreen support or the bmp command.

However, the splashscreen function *only* supports standard BMP images.

This patch adds the documented gzip support.

Signed-off-by: Mark Jackson <mpfj@mimc.co.uk>