package/pkg-kconfig: generate generic help

Currently, as Thomas pointed out [0], the help for kconfig packages is
not consistently used and handled by the different packages.

This commit introduces a generic help text for kconfig packages, that is
based on what the package declares:

  - the list of kconfig editors it supports;

  - whether it is possible to save back the configuration (impossible if
    the package uses an in-tree defconfig file);

  - whether the package actually supports (loading and saving) defconfig
    files, by introducing a new variable a package can set if it does
    not (only busybox is known to be in that case).

That new help helper is only used if the package does not already define
its own help, to be consistent with what we do for other _CMDS.

[0] http://lists.busybox.net/pipermail/buildroot/2021-July/313570.html

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

docs/manual/adding-packages-kconfig.txt

@@ -91,3 +91,6 @@ be set to suit the needs of the package under consideration:
 * +FOO_KCONFIG_DEPENDENCIES+: the list of packages (most probably, host
   packages) that need to be built before this package's kconfig is
   interpreted. Seldom used. By default, empty.
+
+* +FOO_KCONFIG_SUPPORTS_DEFCONFIG+: whether the package's kconfig system
+  supports using defconfig files; few packages do not. By default, 'YES'.