lib: add function check_bootcounter()

Function to validate values from /boot/uboot.cnt

accept 4 arguments:
  magic: bd
  version: 1
  bootcount: any number
  upgrade bit:
    0: not in upgrade
    1: in upgrade

In case if any argument is not defined i.e. passed empty value ""
it is meaning no need to validate that argument

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>