boot_serial: Support sha256, sha384 and sha512

Add support for reporting various sha in image list.
There is always only one sha compiled in, but serial recovery
has been previously hardcoded to support sha256 only.

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: Dominik Ermel <dominik.ermel@nordicsemi.no>
(cherry picked from commit d59ae346)