test: packet_gen: add possibility to define custom L3 headers

Add new -U option to define custom L3 headers. Option string consists
of wanted EtherType followed by a set of field definitions containing
the field name, length, value and a diff value that's added (subtracted
in case of a negative value) to the value element in successive packets.

If given, the custom L3 header overrides the default IP header and any
related options. Additionally, L4 checksums requiring a L3 IP header
won't be calculated.

Signed-off-by: Tuomas Taipale <[email protected]>
Reviewed-by: Matias Elo <[email protected]>