sandbox: Add facility to save/restore sandbox state

It is often useful to be able to save out the state from a sandbox test
run, for analysis or to restore it later to continue a test. Add generic
infrastructure for doing this using a device tree binary file. This is
a flexible tagged file format which is already supported by U-Boot, and
it supports hierarchy if needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Hung-ying Tyan <tyanh@chromium.org>