sd-dhcp: checksum - make endianess-neutral

sd-dhcp: checksum - make endianess-neutral

For efficiency, we group bytes together before adding them up. This
is guaranteed to always work (regardless of the byte order) as long
as the i-th byte in each group lign up with the i-th byte in each
other group.

On big-endian machines this broke when handling the trailing few bytes
which did not make up a full group of 4 bytes. This patch fixes the
problem by explicitly creating a 4 byte zero-padded group out of the
trailing bytes.

Reported and tested by Thomas Ritter <th.ritter@gmx.at>.