I finally found the root of the problem and with that also the solution!
Alltho Google offers a package of allegedly all vendor specific libraries, the containing makefiles list a few (17) libraries that are not included in the package.
To add those libraries to your source folder you either need another hammerhead devices with stock firmware or the system.img file of the ROM.
you can get the libraries from the device like that:
adb pull system/app/OmaDmclient.apk
adb pull system/etc/DxHDCP.cfg
adb pull system/vendor/bin/vss_init
adb pull system/vendor/firmware/discretix/dxhdcp2.b00
adb pull system/vendor/firmware/discretix/dxhdcp2.b01
adb pull system/vendor/firmware/discretix/dxhdcp2.b02
adb pull system/vendor/firmware/discretix/dxhdcp2.b03
adb pull system/vendor/firmware/discretix/dxhdcp2.mdt
adb pull system/vendor/lib/libDxHdcp.so
adb pull system/vendor/lib/libvdmengine.so
adb pull system/vendor/lib/libvdmfumo.so
adb pull system/vendor/lib/libvss_common_core.so
adb pull system/vendor/lib/libvss_common_idl.so
adb pull system/vendor/lib/libvss_common_iface.so
adb pull system/vendor/lib/libvss_nv_core.so
adb pull system/vendor/lib/libvss_nv_idl.so
adb pull system/vendor/lib/libvss_nv_iface.so
if you use the system.img file then mount the image and copy them from there.
Now that we have the missing libraries we need to place them in the vendor directory: vendor/lge/hammerhead/proprietary
and add them to the makefile vendor/lge/hammerhead/device-partial.mk
like that:
vendor/lge/hammerhead/proprietary/libvss_nv_iface.so:system/vendor/lib/libvss_nv_iface.so:lge \
vendor/lge/hammerhead/proprietary/libvss_nv_idl.so:system/vendor/lib/libvss_nv_idl.so:lge \
vendor/lge/hammerhead/proprietary/libvss_nv_core.so:system/vendor/lib/libvss_nv_core.so:lge \
vendor/lge/hammerhead/proprietary/libvss_common_iface.so:system/vendor/lib/libvss_common_iface.so:lge \
vendor/lge/hammerhead/proprietary/libvss_common_idl.so:system/vendor/lib/libvss_common_idl.so:lge \
vendor/lge/hammerhead/proprietary/libvss_common_core.so:system/vendor/lib/libvss_common_core.so:lge \
vendor/lge/hammerhead/proprietary/libvdmfumo.so:system/vendor/lib/libvdmfumo.so:lge \
vendor/lge/hammerhead/proprietary/libvdmengine.so:system/vendor/lib/libvdmengine.so:lge \
vendor/lge/hammerhead/proprietary/libDxHdcp.so:system/vendor/lib/libDxHdcp.so:lge \
now recompile, flash the image and everything should work.