boot: boot_serial: Fix uninitialised variables for upload

Fixes some issues whereby some variables were not initialised,
and undefined values were wrongly used instead

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 b18b91c2)