monitordisk: add event

monitordisk: add event

The current disk usage is interesting and may be worth logging over
time as part of the build statistics. Instead of re-implementing the
code and the configuration option (BB_DISKMON_DIRS), the information
gathered by monitordisk.py is made available to buildstats.bbclass via
a new event.

This has pros and cons:
- there is already a useful default configuration for "interesting" directories
- no code duplication
- on the other hand, users cannot configure recording separately from
  monitoring (probably not that important)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>