package: use the generic _HELP_CMDS for kconfig-based packages

package: use the generic _HELP_CMDS for kconfig-based packages

As Thomas put it:

    The <pkg>_HELP_CMDS variable allows packages using the
    kconfig-package infrastructure to display their specific
    targets related to the handling of their configuration.

    However, it was not consistently used and handled by the
    different packages.

So, this commit switches all the kconfig-based package to use the
generic help helper.

As a consequence:

  - all kconfig packages now advetise their kconfig-related actions,
    where some were previously missing: at91bootstrap3, linux-backports,
    swupdate, xvisor;

  - busybox advertises it does not support defconfig files;

  - the 'foo-savedfconfig' action is no longer advertised: it is to be
    considered an internal implementation detail.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>