buildstats.bbclass: add functionality to collect build system stats

There are a number of timeout and hang defects where
it would be useful to collect statistics about what
is running on a build host when that condition occurs.

This adds functionality to collect build system stats
on a regular interval and/or on task failure. Both
features are disabled by default.

To enable logging on a regular interval, set:
BB_HEARTBEAT_EVENT = "<interval>"
BB_LOG_HOST_STAT_ON_INTERVAL = <boolean>
Logs are stored in ${BUILDSTATS_BASE}/<build_name>/host_stats

To enable logging on a task failure, set:
BB_LOG_HOST_STAT_ON_FAILURE = "<boolean>"
Logs are stored in ${BUILDSTATS_BASE}/<build_name>/build_stats

The list of commands, along with the desired options, need
to be specified in the BB_LOG_HOST_STAT_CMDS variable
delimited by ; as such:
BB_LOG_HOST_STAT_CMDS = "command1 ; command2 ;... ;"

Signed-off-by: Sakib Sajal <sakib.sajal@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>