[PATCH v5] test: packet_gen: add possibility to define custom L3 headers

[TuomasTaipale]

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.

v2:

• Rebased

v3:

• Rebased

v4:

• Addressed comments

v5:

• Fixed field name buffer size
• Added reviewed-by tags