[pxeprefix] Ignore errors from PXENV_FILE_CMDLINE

PXENV_FILE_CMDLINE is an iPXE extension, and will not be supported by
most PXE stacks.  Do not report any errors to the user, since in
almost all cases the error will mean simply "not loaded by iPXE".

Reported-by: Patrick Domack <patrickdk@patrickdk.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/pxeprefix.S

 	movw	$0xffff, ( pxe_parameter_structure + 0x02 )
 	movw	$PXENV_FILE_CMDLINE, %bx
 	call	pxe_call
-	jnc	1f
-	call	print_pxe_error
-	jmp	99f
-1:	/* Check for non-NULL command line */
+	jc	99f  /* Suppress errors; this is an iPXE extension API call */
+	/* Check for non-NULL command line */
 	movw	( pxe_parameter_structure + 0x02 ), %ax
 	testw	%ax, %ax
 	jz	99f