buildroot

poco: add new options for Json, CppParser, MongoDB, PDF and Redis

New options allow to build following components:

* Json
* Redis
* MongoDB
* CppParser
* PDF

This allows to significantly reduce the size of the POCO
libraries. Even though it breaks backward compatibility because those
features were enabled and are now disable by default, Buildroot's
policy is to build the minimal feature-set by default. And users will
get a very clear build failure, which should be easy to fix.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
[Thomas: add an explanation about backward compatibility.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>