package: make *-rebuild and *-reconfigure only rebuild the given package

Originally, the <pkg>-rebuild and <pkg>-reconfigure targets were meant
to restart the build of the package from a given step (build for
<pkg>-rebuild and configure for <pkg>-reconfigure) and then re-create
the entire root filesystem.

However, further discussion from the community has shown that this is
not really the desired behavior: we instead want <pkg>-rebuild and
<pkg>-reconfigure to only take care of rebuilding the given package,
and not the entire root filesystem.

People willing to rebuild this package and the root filesystem can do:

       make <pkg>-rebuild all

[Thomas P: rewrite commit log, since it's not fixing a bug, but
instead changing what was an intended behavior. ]

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>