armbian-next: rootfs: new CLI command for rootfs cache building; further refactorings

- _told ya it would all make sense eventually..._
- introduce new, real, non-alias `rootfs` command in `cli-rootfs` -- this _only_ builds rootfs, nothing else
- no more `main_default_build_single()`; instead `full_build_packages_rootfs_and_image()`
- introduce `do_with_default_build()` wrapper, for things that need host prepared + aggregation to run
- CLI-specific parts of `main_default_build_single()` moved to cli-build
- put prepare/cleanup pair of `prepare_rootfs_build_params_and_trap()` and `trap_handler_cleanup_rootfs_and_image()` into new `trap-rootfs.sh` (this needs further splitting for the image part)
- refactor `get_or_create_rootfs_cache_chroot_sdcard()` into `build_rootfs_only()`, `calculate_rootfs_cache_id()`
- update library with new files

lib/functions/cli/commands.sh

@@ -22,7 +22,9 @@ function armbian_register_commands() {
 		["build"]="standard_build" # implemented in cli_standard_build_pre_run and cli_standard_build_run
 		["distccd"]="distccd"      # implemented in cli_distccd_pre_run and cli_distccd_run
 
-		# shortcuts, see vars set below
+		["rootfs"]="rootfs" # implemented in cli_rootfs_pre_run and cli_rootfs_run
+
+		# shortcuts, see vars set below. the use legacy single build, and try to control it via variables
 		["kernel"]="standard_build"
 		["u-boot"]="standard_build"
 		["uboot"]="standard_build"