efi_loader: EFI_PXE_BASE_CODE_PROTOCOL stub

U-Boot implements the EFI_PXE_BASE_CODE_PROTOCOL because GRUB uses the mode
information for booting via PXE. All function pointers in the protocol were
NULL up to now which will cause immediate crashes when the services of the
protocol are called.

Create function stubs for all services of the protocol returning
EFI_UNSUPPORTED.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>