Deduplicate ether_to_eui64() implementation (#204)

Now when get_first_ethernet() is implemented for both Linux and Solaris,
implementation of ether_to_eui64() function can use this function
get_first_ethernet() and therefore be system independent.

So change implementation of ether_to_eui64() to use get_first_ethernet()
function and move it from Linux and Solaris files to common ipv6cp.c file
where it is used.

Signed-off-by: Pali Rohár <pali@kernel.org>