Changes with possible impact on caller project

safeguard to avoid polution of sstate-cache does not use BASEGEN_ALLOWED_BRANCHES any more (which offered very low protection), but a "permissions" file in sstate-cache dir itself
local.conf can be more easily tuned for project-specific needs using new ci-mkconf-hook.sh
early version of SSTATE_MIRRORS usage caused absolute symlinks to mirrors (in ci-runner filesystem) to be rsync’d, for some yocto versions. They are not sync’d any more, but any such symlink has to be removed manually from the rsync repository (eg. with find -type l -delete)
oe-logs artifacts are now "expire_in: never", much like the job output
buildhistory and prserv support are now optional
copy-on-write sstate handling is now replaced by use of SSTATE_MIRRORS, local.conf in local build tree must be adjusted
location of poky tree is not hardcoded any more, must now be specified through POKY_ENVINIT variable
new rsync-sstate script to download shared state uploaded by the build
log artifacts are now in generic artifacts/ directory rather than in oelogs/

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