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>

Merge the pxe_set_netdev()+pxe_[un]hook_int1a() pattern into a single
pxe_[de]activate() call.

pxe_init_structures() fills in the fields of the !PXE and PXENV+
structures that aren't known until gPXE starts up.  Once gPXE is
started, these values will never change.

Make pxe_init_structures() an initialisation function so that PXE
users don't have to worry about calling it.

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

__cdecl is a misleading name, since it currently encapsulates both
cdecl and regparm(0) attributes.  Rename to __asmcall.

Implement PXENV_RESTART_TFTP.

Move pxe_boot() code to pxe_image.c