boot: bootutil: Refactor erase functionality to fix watchdog feeding

Refactors the erase functionality in bootutil so that it can be used
alongside feeding the watchdog. This has also optimised some
functions out.

Note: This PR has been modified from the upstream commit due to
a merge failure from other changes that are not being brought into
Zephyr 3.7

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 7253f01c)