azure-iot-sdk-c: use github for homepage

Many Buildroot users can not read Dutch. Although there is an English
version of this page, the github page looks more suitable as a link for
the software package itself, and the readme.md file is pretty
informative.

Cc: André Hentschel <nerv@dawncrow.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>