Separate out the concept of "hardware maximum supported frame length"
and "configured link MTU", and limit the latter according to the
former.

In networks where the DHCP-supplied link MTU is inconsistent with the
hardware or driver capabilities (e.g. a network using jumbo frames),
this will result in iPXE advertising a TCP MSS consistent with a size
that can actually be received.

Note that the term "MTU" is typically used to refer to the maximum
length excluding the link-layer headers; we adopt this usage.

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

Add support for EoIB devices as implemented by Xsigo.  Based on the
public (but out-of-tree) Linux kernel drivers at

  https://oss.oracle.com/git/?p=linux-uek.git;a=log;h=v4.1.12-32.2.1

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