fix byteswap in ipudp_checksum and checksum_partial

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Merge pull request #139 from olafhering/olh-dhcp-endian-bugs
objectmodel: removed unused hwaddr interface property
vlan: set link-layer address in newDevice()
vlan: allow to change link-layer address