Basic support for ranges in generator

Basic support for ranges in generator

This commit add initial support for ranges.
Ranges can now be specified using an offset and a range, in a similar
way as random. e.g.:
range=2-16001
range_offset=28
The two previous lines will range the src IP address from x.x.0.2 to
x.x.3e.81 in an IPv4 packet.
The range option will write as many bytes as the bytes needed for
the max (2 bytes in the example above), up to 4 bytes.
Runtime, the generator will iterate through the range in a linear way,
starting from the min.

No check are done whether range overlap themselves or whether they
overlap with randoms.
As of now, no support for command line or script option.

Signed-off-by: Xavier Simonart <simonartxavier@gmail.com>
Change-Id: I98a1757998955f48631a5929bd7534481975eedc