lib: add string to macaddress and reverse function

lib: add string to macaddress and reverse function

As mac-address structure is to be used as a prefix field, two new
functions permit handling mac address in order to convert it. either
from string to internal value, or the reverse operation.
Internal representation of a mac address is a 6 byte char value standing
for the 6 byte value the mac address has.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>