bootutil: Introduce HW rollback protection

- Add image security counter verification (read security counter value
  from the image manifest and compare it against the stored/active
  security counter) as an optional part of the image validation process
  to prevent the restoration of older, potentially vulnerable images.
- This feature can be enabled with the MCUBOOT_HW_ROLLBACK_PROT option.
- Add security counter interface to MCUBoot. If HW rollback protection
  is enabled then the platform must provide a mechanism to store and
  read the security counter value in a robust and secure way.

Change-Id: Iee4961c1da5275a98ef17982a65b361370d2a178
Signed-off-by: David Vincze <david.vincze@arm.com>