[prefix] Reasonable value for lkrn initrd_addr_max

H. Peter Anvin <hpa@zytor.com> sent word that Sergey Vlasov
<vsu@altlinux.ru> discovered gPXE lkrn images fail to load in SYSLINUX
3.70 because we have initrd_addr_max zeroed.  This patch sets the same
value as the Linux kernel.

Also change the header jmp instruction to use a hardcoded opcode value
like Linux does.  Just in case the assembler decides to use a three-byte
instruction instead of the desired two-byte jmp.

src/arch/i386/prefix/lkrnprefix.S

 boot_flag: 
 	.word	0xAA55
 jump:
-	jmp	setup_code
+	/* Manually specify a two-byte jmp instruction here rather
+	 * than leaving it up to the assembler. */
+	.byte	0xeb
+	.byte	setup_code - header
 header:
 	.byte	'H', 'd', 'r', 'S'
 version:
 cmd_line_ptr:
 	.long	0
 initrd_addr_max:
-	.long	0
+	/* We don't use an initrd but some bootloaders (e.g. SYSLINUX) have
+	 * been known to require this field.  Set the value to 2 GB.  This
+	 * value is also used by the Linux kernel. */
+	.long	0x7fffffff
 kernel_alignment:
 	.long	0
 relocatable_kernel: