kernel: mmu: pin/unpin boot sections during boot process

During boot process, the boot sections need to be pinned in
memory to prevent them from being paged out (to avoid
pages being paged out and immediately paged in again).
Once the boot process is completed (just before calling main()),
the boot sections can be unpinned so the memory can be
used for demand paging for paging in data pages.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>