Unify 'ipc.h' and 'ipc_generic.h' across platforms

The distinction between 'ipc.h' and 'ipc_generic.h' is no more. The only
use case for platform-specific extensions of the IPC support was the
marshalling of capabilities. However, this case is accommodated by a
function interface ('_marshal_capability', '_unmarshal_capability'). By
moving the implementation of these functions from the headers into the
respective ipc libraries, we can abandon the platform-specific 'ipc.h'
headers.
pistachio: Remove dead code
base-hw: Adaptation to signal API change

This patch is an interim fix for building base-hw until the signal
implementation is revisited on this platform.
xvfb: Adaptation to timer and framebuffer changes
base-host: Build fixes