boot_serial: Use flash area from bootloader state when possible

Now we have a valid bootloader state in bs_list and bs_set, opening and
closing the flash areas within those routines is no longer needed. This
commit removes superfluous open/close.

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: Thomas Altenbach <thomas.altenbach@legrand.com>
(cherry picked from commit 13edc5fb)