Beside having a `CONFIG_BOARD_REVISION` Kconfig string that represents
our semantic versioning for board revisions, the ZigBee sample requires
an integer representation of it. Instead of conditioning the integer
version on the string version only for that sample and having to touch
the ZigBee sample whenever we add a revision, I decided to have a
board-wide config, so that both string and int versions live close to
each other.
Fixes#138.
