buildstats.bbclass: collect data in the same file.

Previously "at interval" and "on failure" logs were collected
in separate files. Collect both types of logging in the same
file for better analysis.

Introduced new variable which allows different set of commands
to be run by the different logging, interval or failure. The
variables are BB_LOG_HOST_STAT_CMDS_INTERVAL and
BB_LOG_HOST_STAT_CMDS_FAILURE respecteviely.

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