bootcount: Add function wrappers to handle bootcount increment and error checking

bootcount: Add function wrappers to handle bootcount increment and error checking

Those two functions can be used to provide easy bootcount management.

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