nova: extent ec_ctrl in kernel and base-nova

Add possibilities to
* to donate explicitly a SC to an EC running inside the same PD
* to request a re-schedule performed by the kernel

issue #986
nova: set pager and native ec cap for main thread
base: support cmpxchg with different sizes
nova: base-nova specific spinlock replacement
tool: add thread test to autopilot list
nova: support Genode style priority propagation

Read the init documentation for more. This commits enables finally just
priorities for Genode.

Fixes #986