nativesdk-dbus: fix spurious unpackaged files

nativesdk-dbus: fix spurious unpackaged files

When building agl-demo-platform-crosssdk, the package nativesdk-dbus
is required. In the post-install step for the normal package dbus, some
systemd files and symbolic links are added, but this shouldn't happen
when building the nativesdk package, as those added files are not packaged,
thus creating a QA Error (some files are installed in sysroot but not packaged)

Bug-AGL: SPEC-235

Change-Id: Ib4894309355b807d52b34934a2f8a0bdebe3a01a
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>