- introduce generic `prepare_tmpfs_for()`, which manages it's own cleanup/dir removal
- use it for `WORKDIR` (which is `TMPDIR`) and `LOGDIR`
- adapt previous cleanup handlers for those, so they delete their contents but not the dir itself (which might be mounted)
- also: make `ARMBIAN_LOG_CLI_ID` readonly together with other superglobals
- set `XDG_RUNTIME_DIR` & `XDG_RUNTIME_DIR` together with `TMPDIR`
- kernel-make.sh: pass `CCACHE_TEMPDIR` down to Kernel make (thanks @the-Going)