sandbox: Allow reading/writing of RAM buffer

It is useful to be able to save and restore the RAM contents of sandbox
U-Boot either for setting up tests, for later analysys, or for chaining
together multiple tests which need to keep the same memory contents.

Add a function to provide a memory file for U-Boot. This is read on
start-up and written when shutting down. If the file does not exist
on start-up, it will be created when shutting down.

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