[kernel] Move log buffer to its own memory section

In prep for the coming change to keep log entries as a true ring buffer,
move the log buffer from bss into its own memory section.

Related changes in this commit:
- New vm_area_ring, which maps a set of pages twice to allow easy linear
  reading of data from a ring buffer when it wraps around the end.
- logger_init() went away, and the logger ctor is called from
  mem::initialize()
- Instead of an event object, the logger just has a bare wait_queue
- util::counted::from template type changed slightly to allow easy
  conversion from an intptr_t as well as a pointer
- Previously added debugcon_logger code removed - this will be added in
  a separate file in a followup commit

src/kernel/wait_queue.h

@@ -19,6 +19,9 @@ public:
     /// queue lock.
     void add_thread(obj::thread *t);
 
+    /// Block the current thread on the queue.
+    void wait();
+
     /// Pops the next waiting thread off the queue.
     /// Locks the queue lock.
     inline obj::thread * pop_next() {