jpeg: convert to a real package

jpeg is a virtual package, but since it is listed in the dependencies
of other packages, it should obey to all the normal make rules for
packages. Notably, the jpeg-show-depends target is mandatory for the
graph-depends script to work.

Instead to implement such a make target manually, make jpeg a normal
generic-package, except that it doesn't have any source.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>