rootfs.py: fix logic error so that warnings are checked

rootfs.py: fix logic error so that warnings are checked

The following commit caused log checking for warnings not working for RPM
rootfs.

    rootfs.py: ignore "NOTE:" when catching warnings

The problem is that checking for warnings is always skipped because the
following statement is always true.

    if 'log_check' or 'NOTE:' in line:

This patch fixes the above problem so that warning checking in RPM rootfs
can work again.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>