bootcount: Make bootcount magic configurable

Add new Kconfig option, SYS_BOOTCOUNT_MAGIC, to select the boot
counter magic word. This can be useful ie. in case the entire
boot counter register is not usable.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
6 files changed