sandbox: Add CONFIG_OF_HOSTFILE to read FDT from host file

With sandbox it is tricky to add an FDT to the image at build time (or
later) since we build an ELF file, not a plain binary, and the address
space of the whole U-Boot is not accessible in the emulated memory map
of sandbox.

Sandbox can read files directly from the host, though, so add an option
to read an FDT from a host file on start-up.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/README.fdt-control

@@ -142,7 +142,11 @@ join the two:
 
 and then flash image.bin onto your board.
 
-You cannot use both of these options at the same time.
+If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
+startup. This is only useful for sandbox. Use the -d flag to U-Boot to
+specify the file to read.
+
+You cannot use more than one of these options at the same time.
 
 If you wish to put the fdt at a different address in memory, you can
 define the "fdtcontroladdr" environment variable. This is the hex