It is common for system memory maps to be grotesquely unreliable
during POST.  Many sanity checks have been added to the memory map
reading code, but these do not catch all problems.

Skip relocation entirely if called during POST.  This should avoid the
problems typically encountered, at the cost of slightly disrupting the
memory map of an operating system booted via iPXE when iPXE was
entered during POST.  Since this is a very rare special case (used,
for example, when reflashing an experimental ROM that would otherwise
prevent the system from completing POST), this is an acceptable cost.

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

For extremely tight space requirements and specific applications, it is
sometimes desirable to create gPXE images that cannot provide the PXE API
functionality to client programs. Add a configuration header option,
PXE_STACK, that can be removed to remove this stack. Also add PXE_MENU
to control the PXE boot menu, which most uses of gPXE do not need.

Signed-off-by: Marty Connor <mdc@etherboot.org>