hw: fix bug in scheduler timing

By now the scheduling timer was only refreshed for a new scheduling timeout
when the choosen scheduling context has changed. But we want it to be refreshed
also when the scheduled context yields without an effect to the schedulers
choice (this is the case e.g. when the idle thread gets a scheduling timeout
or a thread yields without any competitor in its priority band).

ref #1151
hw: get a thread cap in Thread_base constructor

Previously this was not done before Thread_base::start(..) in
base-hw as it was not needed to have a valid cap that early. However,
when changing the affinity of a thread we need the cap to be valid
before Thread_base::start(..).

fix #1151