Create the context area RM session at startup

With this patch, the thread context area RM session gets created at
program startup to have the thread context area's virtual address range
reserved right from the beginning.

Fixes #734.

This patch implies that all Genode processes try to create an RM
session. So a route to the RM service must be present even for processes
that have only a single thread. Hence, the patch contains an update of
affected components.
hw & core: provide env_context_area_rm_session()

Ref #734
Add missing route to lx_uid.run
Add missing route to chroot_loader test

Additionally, we do the cleanup of the remains of a previously failed
run prior creating the run directory. Otherwise, the directly creation
may fail because of a pending bindmount.
Adapt chroot.run to changed timer