During development it is often handy to change the config.h options from
their defaults, for example to enable debugging features.

To prevent accidental commits of debugging config.h changes, mdc
suggested having a config-local.h that is excluded from source control.
This file acts as a temporary config.h and can override any of the
defaults.

This commit is an attempt to implement the config-local.h feature.

The config.h file now has the following as its last line:
/* @TRYSOURCE config-local.h */

The @TRYSOURCE directive causes config-local.h to be included at that
point in the file.  If config-local.h does not exist, no error will be
printed and parsing will continue as normal.  Therefore, mkconfig.pl is
"trying" to "source" config-local.h.

The GDBSYM config.h option was an attempt at QEMU GDB debugging.  I have
removed the code since it is unused and may confuse people wanting to
use the GDB stub.

IPv4 may not always be dragged into the link, so add a specific
configuration option for it.

be propagated to master.

	Timer subsystem initialization code in core/timer.c

	Split the BIOS and RTDSC timer drivers from i386_timer.c

	Split arch/i386/firmware/pcbios/bios.c into the RTSDC
	timer driver and arch/i386/core/nap.c

	Split the headers properly:
		include/unistd.h - delay functions to be used by the
					gPXE core and drivers.

		include/gpxe/timer.h - the fimer subsystem interface
					to be used by the timer drivers
					and currticks() to be used by
					the code gPXE subsystems.

		include/latch.h	- removed
		include/timer.h - scheduled for removal. Some driver
					are using currticks, which is
					only for core subsystems.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>

Add IMAGE_PXE to use the new image format framework.  "kernel pxelinux.0"
now works.

Add scripts as an image format (since it's trivial to do).

can't modify it).

"boot" command to attempt booting from within the command shell, fall back
to shell if boot fails for any reason.

the new UDP API yet.

need for a CONFIG_RELOCATE.

Change from using #if to #ifdef, since #if can't cope with string
constants.

Removed CONFIG_{PCI,ISA,PCMCIA}, since buses are now always compiled and
linked in only if required by the individual driver objects.