loaders: Add ByteUtils.

ByteUtils provide GP_FWrite() and GP_FRead() functions
to ease reading and writing image file headers.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: BMP: Experimental BMP writer for RGB888.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>