Add device assignment related flags

So far we have the following 2 flags:
* RELEASE_AVF_ENABLE_DEVICE_ASSIGNMENT - controls the whole device
  assignment feature. This flag depends on the
  RELEASE_AVF_ENABLE_VENDOR_MODULES one.
* RELEASE_AVF_ENABLE_VENDOR_MODULES - controls whether it is possible to
  load vendor modules in guest VMs.

Bug: 298007909
Test: builds
Change-Id: I73eb69f2ea3a4f98a406dc02cb695438428b2658