[yocto] Dependencies on other file system types for custom file system type

[Eric Grunt]

> Notice how the function names for squashfs are:
>
> do_image_squashfs_xz
>
> not
>
> do_image_squashfs-xz
>
> This is because shell functions/variables can't have "-" in their name.
> You need to change to us IMAGE_CMD_squashfs_xz_ubi.

I tried to basically change all "-" in the fs and variable names into "_",
but this didn't change the behaviour.
What I do not understand:
The filesystem type squashfs-xz (included in poky) uses "-",
among others also for the IMAGE_CMD name ("IMAGE_CMD_squashfs-xz").
Only the dependencies to squashfs tools are using "_":
do_image_squashfs_xz[depends] += "squashfs-tools-native:do_populate_sysroot"

In the case of squashfs_xz_ubi the IMAGE_TYPEDEP_ dependency
to sqashfs-xz is ignored.
(experimentally trying to change that dependency to squashfs
(a name without "-")  does not help)