meta-titanium: machine conf: don't force preferred kernel
meta-titanium: machine conf: don't force preferred kernel

This allows overwriting this value in site.conf or local.conf

Co-authored-by: Joerg Hofrichter <64f05fd09614173ff715799ea05b4b0d8fbec0da@ni.com>
Signed-off-by: Steven Koo <2be81fa51a694756e39f0ec857909538bdef4f38@ni.com>

EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus