log: refactor code, move the code to get default log level
by parsing environment variables to getDefaultLogLevel()

Change-Id: If6d6b70855600731e40158b947a776ec559d5808
log: refactor code, define interface of backends, and add two
backends named console and deepinlog

Change-Id: Ie9184aebc5584e6c7a89943e9f8e18319b821140