packages: make providers declare what they provide

Fixes:
    http://autobuild.buildroot.org/results/285/2851069d6964aa46d26b4aabe7d84e8c0c6c72ce
    http://autobuild.buildroot.net/results/9b7/9b7870354d70e27e42d3d9c1f131ab54706bf20e


    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>