mtd-utils: add ubihealthd to nand-utils

mtd-utils: add ubihealthd to nand-utils

Add ubihealthd to the nand-utils package, auto-create UCI config for
each UBI device and launch the daemon on boot.

The default time interval between scrubbing a random PED is 120 seconds
which means that a fully used 128 MiB flash chip gets scrubbed in about
a day and a half. The interval can be adjusted in UCI using the
'interval' option.

Suggested-by: Rodrigo Balerdi <lanchon@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://github.com/openwrt/openwrt/pull/16973
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>