UNPACKDIR is not set on scarthgap

[philippgl]

Describe the bug

When compiling some merged #149 I get the following funny looking error;
...
| /home/pglaser/git/bif/scarthgap/radxa-zero/tmp/work/radxa_zero-poky-linux/shadow-securetty/4.6/temp/run.do_configure.808231: 140: cannot create /securetty: Permission denied
...

I skipped the rest of the template, b/c I think, the UNPACKDIR was introduced on master and does not exist in the scarthgap branch.
I would suggest adding a scarthgap branch based on the previous head and removing scarthgap from the
LAYERSERIES_COMPAT_meson

[superna9999]

Sure, I'll do that

[superna9999]

Done!