toaster: buildinfohelper: Use correct way to get message from LogMessage

toaster: buildinfohelper: Use correct way to get message from LogMessage

Use the correct method to get a message value from the LogMessage object
rather than constructing it ourselves which is not recommended. This
causes an exception when the msg contains a '%' such as when there are
wildcards in file names (something2.%.bbappends)

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