Also, preserve and restore current configuration, so I don't get mad
when checking with MAKEALL during my tests for new feature.

The rationale for this is that wrpc-sw will soon select different ppsi
choices according to its own configuration, through $(PPSI-CFG-y).
Actually, it already does for the host process build.

MAKEALL must obey this choice, so we trim ppsi configuration
each time we reconfigure wrpc (removing the file would spit an
error instead).

Then, our Makefile forces a ppsi reconfig if ppsi/.config is missing
*or* empty.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>