[sw,silicon_creator] Make Silicon Creator code top agnostic

Some silicon creator libraries (e.g. the [otbn_boot_services](https://github.com/lowRISC/opentitan/blob/master/sw/device/silicon_creator/lib/otbn_boot_services.c) library) have hard coded dependencies on specific tops.

In order for those libraries to be shared between top's ROMs and ROM_EXT implementation, we should have a generic framework to allow top software definitions/hooks that could be called from the shared libraries.

cc @luismarques @pamaury