boot_serial: Replace cbor auto-generated code with zcbor functions

Replaces the auto-generated decoding/encoding files with inline code
for encoding/decoding cbor data structures, this adds the benefit of
allowing the elements to be in any order and reduces code size. To
accommodate this, zcbor_bulk has been imported from Zephyr.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>