efi_loader: use well-known guid for auto-created boot option

The boot option automatically created by efibootmgr is identified
by the special guid appended in the optional data of boot option.
The same mechanism is implemented in the EDK II reference
implementation, it uses the different guid from the one currently
U-Boot uses.
The guid indicating auto-created boot option is not defined in the
UEFI specification, but some userspace tools such as 'efivar' package
are aware of the guid used in EDK II as auto-created boot option.

So let's use the same guid as EDK II reference implementation.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>