mel: quiet a couple meta-systemd bbappend warnings

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
(cherry picked from commit 9259e0d434dd8acfc18fab17266dee3c8789855a)

Conflicts:

	conf/distro/include/mel.conf

Signed-off-by: Michael Brown <mw_brown@mentor.com>
guile-native: drop no longer needed append

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
(cherry picked from commit f9106ba17992d4fabc34ace5446048d379f77794)

Signed-off-by: Michael Brown <mw_brown@mentor.com>
layer.conf: support per-layer bbappends

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
(cherry picked from commit 2d26db9bba123d380457fd8ab9b0d8fdb6619552)

Signed-off-by: Michael Brown <mw_brown@mentor.com>