bitbake.conf/image.bbclass: Move image type information into image_types.bbclass
Image generation code in .conf files is hard to read as it needs to be
single line. By moving this to a separate class, multiline functions
can be used instead improving readability. It also declutters
bitbake.conf.
There is no real functional change with this patch but it highlights
the need for improvements in places such as the IMAGE_EXTRA_OPTION
ext* specific variable which makes no sense.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
