package/jpeg: convert to the virtual-package infrastructure

Since this package is implemented via a choice rather than the usual
separate-package providers, we only need BR2_PACKAGE_HAS_JPEG to be
always defined when the 'jpeg' package is selected.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Mike Zick <minimod@morethan.org>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>