base: add flex page iterator

Issue #8
nova: support noreturn attribute for syscall reply
nova: update syscall bindings
nova: use kernel branch with vCPU fpu support

branch r4 based on latest vanilla kernel of @udosteinberg
nova: add feature to yield a thread

used by virtualbox intensively
nova: fix utcb layout for 32bit

Makes virtualbox as 32bit VMM working
ports: add VirtualBox as VMM for Genode

Fixes #1055