[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.cpp

@@ -141,6 +141,11 @@ vm_area_open::get_page(uintptr_t offset, uintptr_t &phys)
     return page_tree::find_or_add(m_mapped, offset, phys);
 }
 
+void
+vm_area_open::add_existing(uintptr_t offset, uintptr_t phys)
+{
+}
+
 
 vm_area_guarded::vm_area_guarded(uintptr_t start, size_t buf_pages, size_t size, vm_flags flags) :
     m_pages {buf_pages + 1}, // Sections are N+1 pages for the leading guard page
@@ -179,4 +184,20 @@ vm_area_guarded::get_page(uintptr_t offset, uintptr_t &phys)
     return vm_area_open::get_page(offset, phys);
 }
 
+vm_area_ring::vm_area_ring(size_t size, vm_flags flags) :
+    vm_area_open {size * 2, flags},
+    m_bufsize {size}
+{
+}
+
+vm_area_ring::~vm_area_ring() {}
+
+bool
+vm_area_ring::get_page(uintptr_t offset, uintptr_t &phys)
+{
+    if (offset > m_bufsize)
+        offset -= m_bufsize;
+    return vm_area_open::get_page(offset, phys);
+}
+
 } // namespace obj