[efi] Add efi_strerror()

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.

src/interface/efi/efi_strerror.c

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2008 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 <stdio.h>
+#include <gpxe/efi/efi.h>
+
+/** @file
+ *
+ * gPXE error message formatting for EFI
+ *
+ */
+
+/**
+ * Format EFI status code
+ *
+ * @v efirc		EFI status code
+ * @v efi_strerror	EFI status code string
+ */
+const char * efi_strerror ( EFI_STATUS efirc ) {
+	static char errbuf[32];
+
+	if ( ! efirc )
+		return "No error";
+
+	snprintf ( errbuf, sizeof ( errbuf ), "Error %lld",
+		   ( unsigned long long ) ( efirc ^ MAX_BIT ) );
+	return errbuf;
+}