efi_loader: Separate device path into its own header

These functions are useful for the EFI app. As a first step towards
making these available outside lib/efi_loader, create a separate header
file and include it where needed. Add proper comments to the functions,
since many are missing at present.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

test/lib/efi_device_path.c

@@ -5,6 +5,7 @@
  * Copyright (c) 2020 Heinrich Schuchardt <xypron.glpk@gmx.de>
  */
 
+#include <efi_device_path.h>
 #include <efi_loader.h>
 #include <test/lib.h>
 #include <test/test.h>