efi_loader: setting boot device

Up to now the bootefi command used the last file loaded to determine the
boot partition. This has led to errors when the fdt had been loaded from
another partition after the EFI binary.

Before setting the boot device from a loaded file check if it is a PE-COFF
image or a FIT image.

For a PE-COFF image remember address and size, boot device and path.

For a FIT image remember boot device and path.

If the PE-COFF image is overwritten by loading another file, forget it.

Do not allow to start an image via bootefi which is not the last loaded
PE-COFF image.

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