manual: add documentation for kconfig-package

This patch adds documentation for the new kconfig-package infrastructure to
the manual.

Note that due to the simplicity of the infrastructure, the documentation
is not split in a 'tutorial' and a 'reference', like for the other
infrastructures. Instead, the usage is described in one section.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
[me: slight grammar fix 'copy -back- the configuration +back+ to...']
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>