armbian-next: u-boot: use git worktree for u-boot; full mainline clone for bare

- share a single `.git` for all u-boots
- all fetches are done against it, maximizing cache hit ratio drastically
- also reduces the size of each working copy by more than 70%
- split uboot's git stuff into uboot-git.sh
- split `uboot_prepare_git()` from `compile_uboot()`

lib/functions/main/config-prepare.sh

@@ -114,7 +114,7 @@ function prepare_and_config_main_build_single() {
 		IMAGE_TYPE=user-built
 	fi
 
-	export BOOTSOURCEDIR="${BOOTDIR}/$(branch2dir "${BOOTBRANCH}")"
+	export BOOTSOURCEDIR="u-boot-worktree/${BOOTDIR}/$(branch2dir "${BOOTBRANCH}")"
 	[[ -n $ATFSOURCE ]] && export ATFSOURCEDIR="${ATFDIR}/$(branch2dir "${ATFBRANCH}")"
 
 	export BSP_CLI_PACKAGE_NAME="armbian-bsp-cli-${BOARD}${EXTRA_BSP_NAME}"