flavours: add reference to oniro-bundle-base recipe

The oniro-bundle-base recipe can be used to build a RAUC update bundle
corresponding to the oniro-image-base. By including this in the conf-notes.txt
file we surface this information where more users will see it.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>