libuecc: use shared instead of static library

libuecc: use shared instead of static library

This patch prepares for another future package (ecdsautils),
which builds multiple binaries all linked to libuecc.

The changes are a direct copy of [1]. The original commit
message was:

> commit cb2ecbfdf0c478568a28aacb99d30fd6ee5c0dd1
> From: Matthias Schiffer <mschiffer@universe-factory.net>
> Date: Tue, 3 May 2016 21:33:34 +0200
> Subject: libuecc: use shared instead of static library
>
> Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

[1]: https://github.com/freifunk-gluon/gluon/blob/cb2ecbfdf0c478568a28aacb99d30fd6ee5c0dd1/patches/packages/openwrt/0007-libuecc-use-shared-instead-of-static-library.patch

Signed-off-by: Dominik Menke <dom@digineo.de>