boot_serial: cbor_encode: Fix encoding data length

Change fixes encoding data length and adds support for big endian
byte ordering. According to specification, data length can be
encoded either on 1, 2, 4 or 8 bytes.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>
(cherry picked from commit 4960d12c)
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>