[efi] Use elf2efi utility in place of efilink

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.

src/arch/x86/prefix/efiprefix.c

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2009 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include <stdlib.h>
+#include <gpxe/efi/efi.h>
+
+/**
+ * EFI entry point
+ *
+ * @v image_handle	Image handle
+ * @v systab		System table
+ * @ret efirc		EFI return status code
+ */
+EFI_STATUS EFIAPI _start ( EFI_HANDLE image_handle,
+			   EFI_SYSTEM_TABLE *systab ) {
+	EFI_STATUS efirc;
+
+	/* Initialise EFI environment */
+	if ( ( efirc = efi_init ( image_handle, systab ) ) != 0 )
+		return efirc;
+
+	/* Call to main() */
+	return RC_TO_EFIRC ( main () );
+}