sim: Generalize test image dumping

Decode the MCUBOOT_DEBUG_DUMP environment variable into a series of
comma-separated strings, and create debug dumps for any test that
matches one of these strings.  Also, the special string 'all' will match
every test, causing every image under test to be dumped.

The dumped images will contain an atomic counter, so that all of the
generated files will be unique for a given run.

Signed-off-by: David Brown <david.brown@linaro.org>