armbian-next: kernel: use git worktree & kernel.org clone bundles for kernel working copies

- now we've a single `.git` for all kernels with all branches; a "master" cache
- back to full stable git bundle usage from kernel.org
- bye bye, "gitballs"
- split kernel-related git stuff into `kernel-git.sh`
- introduce `ARMBIAN_MOUNTPOINTS_DEPRECATED` @TODO actually use for cleaning non-Docker
- add `axel` hostdep for multi-conn download of bundle, which comes from a CDN.
- deprecate old mountpoints for gitballs and linux-kernel
- move cleaning of old sources/kernel to mountpoints code

lib/functions/main/config-prepare.sh

@@ -157,11 +157,11 @@ function prepare_and_config_main_build_single() {
 		fi
 
 		# Default LINUXSOURCEDIR:
-		export LINUXSOURCEDIR="linux-kernel/${KERNEL_MAJOR_MINOR}__${LINUXFAMILY}/${ARCH}"
+		export LINUXSOURCEDIR="linux-kernel-worktree/${KERNEL_MAJOR_MINOR}__${LINUXFAMILY}__${ARCH}"
 
 		# Allow adding to it with KERNEL_EXTRA_DIR
 		if [[ "${KERNEL_EXTRA_DIR}" != "" ]]; then
-			export LINUXSOURCEDIR="linux-kernel/${KERNEL_MAJOR_MINOR}__${LINUXFAMILY}_${KERNEL_EXTRA_DIR}/${ARCH}"
+			export LINUXSOURCEDIR="${LINUXSOURCEDIR}__${KERNEL_EXTRA_DIR}"
 			display_alert "Using kernel extra dir: '${KERNEL_EXTRA_DIR}'" "LINUXSOURCEDIR: ${LINUXSOURCEDIR}" "debug"
 		fi
 	else