boot: replace boot_is_version_sufficient() function

Replace the boot_is_version_sufficient() function with a new,
boot_version_cmp() function which is more straightforward and
distinguishes the 3 possible results of the comparison.
Update the error handling accordingly.

Change-Id: Ie7d4f892dc3df2c6ef82769c4d0676965b75b7b8
Signed-off-by: David Vincze <david.vincze@linaro.org>