boot_serial: Avoid re-initializing state in boot_image_validate_encrypted

A valid bootloader state was needed to validate encrypted images, so the
boot_image_validate_encrypted (only called from bs_list and bs_set) was
allocating and initializing a minimal state with the required content.
Now bs_list and bs_set have a valid bootloader state, the latter can be
given to boot_image_validate_encrypted, avoiding two bootloader state
allocations.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>