busybox: Save shell history into /tmp

busybox: Save shell history into /tmp

Enable the busybox feature to save shell command history.
(.apk size increase 0.5 kB)

To prevent flash wear,
* save history only at the exit from a shell session, and
* set /tmp as the default location for the history file. The history
  is kept on ramdisk until a reboot, when the history is then lost.

If the user wants to save history onto flash, he can change the location
definition in /etc/profile.d/busybox-history-file.sh

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Link: https://github.com/openwrt/openwrt/pull/17179
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>