psmisc: Set ALTERNATIVE for pstree to resolve conflict with busybox

psmisc: Set ALTERNATIVE for pstree to resolve conflict with busybox

If pstree in busybox is enabled there is a conflict with pstree from
psmisc resulting in:

  do_rootfs: Postinstall scriptlets of ['busybox'] have failed. If
  the intention is to defer them to first boot, then please place
  them into pkg_postinst_ontarget:${PN} ().
  Deferring to first boot via 'exit 1' is no longer supported.

And more detailed in do_rootfs.log:

  update-alternatives: Error: not linking [...]/rootfs/usr/bin/pstree to /bin/busybox.nosuid since [...]/rootfs/usr/bin/pstree exists and is not a link

On order to fix this set ALTERNATIVE:pstree accordingly.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>