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>

cmd/efidebug.c

@@ -8,6 +8,7 @@
 #include <charset.h>
 #include <command.h>
 #include <dm/device.h>
+#include <efi_device_path.h>
 #include <efi_dt_fixup.h>
 #include <efi_load_initrd.h>
 #include <efi_loader.h>