piggy.o would be build for every time barebox was built
This had the sideeffect that the image(s) would
always be rebuilt despite no changes

Fix this by adding piggy.o to targets
and avoid an extra command in the rule to create .pblb files

The patch includes the removal of a stray assignment

Fixes: 2078438662 ("Add multi images support")
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>