net: Always build the string_to_enetaddr() helper

Part of the env cleanup moved this out of the environment code and into
the net code. However, this helper is sometimes needed even when the net
stack isn't included.

Move the helper to lib/net_utils.c like it's similarly-purposed
string_to_ip(). Also rename the moved function to similar naming.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reported-by: Ondrej Jirman <megous@megous.com>
diff --git a/net/eth_legacy.c b/net/eth_legacy.c
index 41f5263..5d6b0d7 100644
--- a/net/eth_legacy.c
+++ b/net/eth_legacy.c
@@ -117,7 +117,7 @@
 			switch (op) {
 			case env_op_create:
 			case env_op_overwrite:
-				eth_parse_enetaddr(value, dev->enetaddr);
+				string_to_enetaddr(value, dev->enetaddr);
 				eth_write_hwaddr(dev, "eth", dev->index);
 				break;
 			case env_op_delete: