efi_loader: Separate device path into its own header

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>