buildroot

Makefile: add missing targets to noconfig_targets and nobuild_targets

currently some buildroot targets fails (list-defconfigs,
graph-build, etc), if there is an issue with configuration.
For example, enabling uboot package without providing custom
version name results in failing of various targets.

Signed-off-by: Rahul Jain <Rahul.Jain@imgtec.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: as suggested by Arnout, added printvars and savedefconfig to
nobuild_targets.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>