yocto-compat-layer.py: avoid adding layers more than once
add_layer_dependencies() might get called more than once, or one of
the layer dependencies might already be present. The function should
not add layers again because doing so can cause warnings like:
WARNING: Duplicate inclusion for .../meta-openembedded/meta-oe/conf/distro/include/meta_oe_security_flags.inc in .../meta-openembedded/meta-oe/conf/layer.conf
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
