uclibc: convert to kconfig-package infrastructure

This patch converts the uclibc package to the new kconfig-package
infrastructure, thus removing code duplication and ensuring a consistent
behavior of kconfig packages.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>