Adds a configurable version string to a known location at the end of mboot
flash section. Also stores the options mboot was built with, eg usb and
which filesystems are supported.
A board can override the defaults, or disable the version string entirely
by setting MBOOT_VERSION_ALLOCATED_BYTES=0.
Signed-off-by: Victor Rajewski <victor@allumeenergy.com.au>
