af-main: Bug fixes

 - Fix issue in handling signature of widgets.
 - Fix access to system API afm-main.

This change include the commits:

  * 8c7132a Creates systemd service for APIs
  * d8d819b wgtpkg-digsig: Add flag to accept/refuse a WGT without signature
  * f97f8d2 wgtpkg-digsig: Fix digital signature check

Bug-AGL: SPEC-1592
Bug-AGL: SPEC-1590

Change-Id: I0ecc19466180799e23b7863eb85755d9e4c2894f
Signed-off-by: José Bollo <jose.bollo@iot.bzh>