libv4l: properly error out when one of the make step fails

In libv4l.mk, if there are multiple elements in $(LIBV4L_DIRS_y), they
are built in order, one after the other. However, our loop construct
doesn't take into account the fact that we should error out if one of
the steps failed.

A good illustration is having BR2_PACKAGE_LIBV4L_DECODE_TM6000 and
BR2_PACKAGE_LIBV4L_V4L2_CTL enabled. The build of decode-tm6000 will
fail, but the build will happily continue without stopping in libv4l.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>