babeltrace: append, don't replace

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
liburcu: drop recipe, upstream has it

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Merge pull request #44 from kergoth/fixup-lttng-bits

lttng: don't override recipes from oe-core