[yocto] nested meta layers, how can/should it be done?

[Hans Beckérus]

Hi. I wish to write a custom meta layer that should add on-top of an
existing bsp for eg. kernel and u-boot builds.
But I want the custom layer to use .bbappend for some standard
packages and also add some custom packages.
My guess is that it might cause problems for the standard packages
(since they are not unique) due to the MACHINE will be pointed out as
the base bsp machine config, and the custom bsp will not define a
machine on its own. Does this mean that I will have to define this new
custom layer as a distro? I am worried that .bbappend stuff will never
be executed since nothing will actually point to my custom layer for
package recipes unless they are unique. What is the correct way to
handle this use-case?

Hans