rootfs.py: Use one way to exclude lines in _log_check_error()

rootfs.py: Use one way to exclude lines in _log_check_error()

Before there were three different ways to exclude a line from being
searched for error messages in _log_check_error(). Now there is only
one: an array of regular expressions. This should make it easy to add
more excludes if nedded.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>