m4: simplify path substitutions in ptest

m4: simplify path substitutions in ptest

There's no need to setup a fake 'source directory' and
redirect the tests to it; rather it's possible to point
them directly to ptest directory (this needs to be done
twice, in Makefile substitutions, and as a C define during
ptest compilation).

This also eliminates the assumption that S and B are in the
same directory (no longer true when S is in UNPACKDIR).

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>