package: add selection between libjpeg and jpeg-turbo

Introduce a virtual 'jpeg' package, which pulls in either libjpeg or
jpeg-turbo depending on a choice selection.

Rename jpeg package to libjpeg so we can reuse 'jpeg' for the virtual
package, making the change transparent to existing users and all the
packages using libjpeg.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>