efi_loader: move EFI_LOAD_FILE2_PROTOCOL_GUID

The EFI_LOAD_FILE_PROTOCOL_GUID and EFI_LOAD_FILE2_PROTOCOL_GUID are needed
to complement the implementation of the LoadFile() boot service.

Remove a duplicate declaration of a variable for the
EFI_LOAD_FILE2_PROTOCOL_GUID.
Move the remaining declaration to efi_boottime.c.
Add a variable for the EFI_LOAD_FILE_PROTOCOL_GUID.

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