bootcount: Rewrite autoboot to use wrapper functions from bootcount.h

The code has been refactored to use common wrappers from bootcount.h
header.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Alex Kiernan <alex.kiernan@gmail.com>