busybox: support only one version

busybox: support only one version

This commit removes the version selection for the busybox
package. Busybox is very well maintained, and bugs are typically fixed
in a timely fashion. Moreover, regressions are fairly unlikely in this
very stable and well-tested tool.

Therefore, there isn't a very compelling reason to have a version
selection for Busybox since we don't accept such a version selection
for the vast majority of other packages, unless there is a strong
reason to do so.

Consequently, this commit:

 * Removes the 1.19.4, 1.20.2 and 1.21.1 Busybox versions, patches and
   default configuration file.

 * Moves the 1.22.1 patches from package/busybox/1.22.1 to just
   package/busybox/ like all other packages.

 * Renames the default 1.22.1 configuration file to just
   busybox.config.

 * Adapts the busybox.mk makefile to encode the current version to
   use.

 * Adds appropriate options to Config.in.legacy. However, even though
   the BR2_BUSYBOX_VERSION_1_22_X is removed, we don't add a
   Config.in.legacy option for it, since it would cause a legacy
   warning for virtually *all* users as most people are currently
   using 1.22.x.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>