fbv: Allow selection of supported image formats.

This removes fbv's compulsory dependency on either libpng and/or libjpeg and/or libungif.
The user can choose which compressed image format's fbv should support.
By default PNG, JPEG and GIF are selected.
Note that BMP format support will still always be included.

[Peter: Drop (unneeded) png specific libs]
Signed-off-by: Arn R <arnerro@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>