pblx-* has been removed in 5a1a5ed253 ("ARM: images: use piggydata")
and using it (e.g. because a patchset was rebased) results in a non-obvious:

	make[1]: *** No rule to make target 'images/*.pblx.*', \
	needed by 'images/barebox-*.img'.  Stop.

Avoid this by explicitly failing with an error message whenever pblx-y
or pblx- has been defined.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>