buildinfohelper: Make sure we use the orm defined value for loglevel

We need to consistently use LogMessage.INFO/WARNING/ERROR to make sure toaster knows
how to categories these rather than passing in the "raw" loglevel value
which in best case comes from python logging but worst case any value.

[YOCTO 6885]

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>