[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/objects/vm_area.h

@@ -126,6 +126,10 @@ public:
 
     virtual bool get_page(uintptr_t offset, uintptr_t &phys) override;
 
+    /// Tell this VMA about an existing mapping that did not originate
+    /// from get_page.
+    void add_existing(uintptr_t offset, uintptr_t phys);
+
 private:
     page_tree *m_mapped;
 };
@@ -178,4 +182,25 @@ private:
     block_allocator m_stacks;
 };
 
+
+/// Area that maps its pages twice for use in ring buffers.
+/// Cannot be resized.
+class vm_area_ring :
+    public vm_area_open
+{
+public:
+    /// Constructor.
+    /// \arg size  Virtual size of the ring buffer. Note that
+    ///            the VMA size will be double this value.
+    /// \arg flags Flags for this memory area
+    vm_area_ring(size_t size, vm_flags flags);
+    virtual ~vm_area_ring();
+
+    virtual bool get_page(uintptr_t offset, uintptr_t &phys) override;
+
+private:
+    size_t m_bufsize;
+    page_tree *m_mapped;
+};
+
 } // namespace obj