Changes

osmocom.construct: Bytes + GreedyBytes with automatic hexstring-conversion

This is a convenience version of the Bytes / GreedyBytes construct
that accepts not only a bytes/bytearray object, but also a hex-encoded
string within the encoder.

Related: OS#6774
Change-Id: I59f500c925848872a7fa38d6dbf3d6ea72bc3a90