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>

doc/uefi/uefi.rst

@@ -59,13 +59,10 @@ Below you find the output of an example session starting GRUB::
     120832 bytes read in 7 ms (16.5 MiB/s)
     => bootefi ${kernel_addr_r} ${fdt_addr_r}
 
-The bootefi command uses the device, the file name, and the file size
-(environment variable 'filesize') of the most recently loaded file when setting
-up the binary for execution. So the UEFI binary should be loaded last.
-
-The environment variable 'bootargs' is passed as load options in the UEFI system
-table. The Linux kernel EFI stub uses the load options as command line
-arguments.
+When booting from a memory location it is unknown from which file it was loaded.
+Therefore the bootefi command uses the device path of the block device partition
+or the network adapter and the file name of the most recently loaded PE-COFF
+file when setting up the loaded image protocol.
 
 Launching a UEFI binary from a FIT image
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~