net: add function to convert string to ethernet address

Make a function that can be used in place of eth_aton_r
to convert a string to rte_ether_addr. This function
allows both byte (xx:xx:xx:xx:xx:xx) and word (XXXX:XXXX:XXXX)
format and has the same lack of error handling as the original.

This also allows ethdev to no longer have a hard dependency
on the cmdline library.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>