[kernel] Add HPET support, create clock class

Create a clock class which can be queried for current timestamp in
nanoseconds. Also implements a simple HPET class as one possible clock
source.

Tags: time

src/kernel/scheduler.cpp

@@ -311,6 +311,7 @@ scheduler::schedule()
 		m_blocked.push_back(m_current);
 	}
 
+	clock::get().update();
 	prune(++m_clock);
 
 	priority = 0;