[iobuf] Relax alignment requirement for small I/O buffers

[iobuf] Relax alignment requirement for small I/O buffers

iPXE currently aligns all I/O buffers on a 2kB boundary.  This is
overkill for transmitted packets, which are typically much smaller
than 2kB.

Align I/O buffers on their own size.  This reduces the alignment
requirement for small buffers, while preserving the guarantee that I/O
buffers will never cross boundaries that might cause problems for some
DMA engines.

Signed-off-by: Michael Brown <mcb30@ipxe.org>