sandbox: test: Provide an easy way to use the other FDT

sandbox: test: Provide an easy way to use the other FDT

Add a test flag which indicates that the 'other' FDT should be set up
ready for use. Handle this by copying in the FDT, unflattening it for
livetree tests. Free the structures when the tests have run.

We cannot use the other FDT unless we are using live tree or
OFNODE_MULTI_TREE is enabled, since only one tree is supported by the
ofnode interface in that case. Add this condition into
ut_run_test_live_flat() and update the comments.

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