boot: bootutil: Move erase function location

Moves the erase function from loader to bootutil misc to account
for MCUboot modes that do not include the main loader file

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 f6e8e88a)