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

elf2efi converts a suitable ELF executable (containing relocation
information, and with appropriate virtual addresses) into an EFI
executable.  It is less tightly coupled with the gPXE build process
and, in particular, does not require the use of a hand-crafted PE
image header in efiprefix.S.

elf2efi correctly handles .bss sections, which significantly reduces
the size of the gPXE EFI executable.

EFI_STATUS is defined as an INTN, which maps to UINT32 (i.e. unsigned
int) on i386 and UINT64 (i.e. unsigned long) on x86_64.  This would
require a cast each time the error status is printed.

Add efi_strerror() to avoid this ickiness and simultaneously enable
prettier reporting of EFI status codes.

We now have two implementations for the timer API: one using the
time-of-day counter at 40:70 and one using RDTSC.  Both make use of
timer2_udelay().

Timers are sometimes required before the call to initialise(), so we
cannot rely on initialise() to set up the timers before use.

Also fix a potential integer overflow issue in generic_currticks_udelay()

Add missing comments to timer code.

Lock system if no suitable timer source is found.

Fix initialisation order so that timers are initialised before code that
needs to use them.

Replace a printf with a DBG in timer_rtdsc.c
Replace a printf in timer.c with assert
Return proper error codes from timer drivers

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

	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>

Kill off use of etherboot.h outside drivers/net.

stack or INT 13 emulator etc.