feat(core): add ROOTFS_TYPE=nfs-root for full network boot

New rootfs type for full network boot: the only thing on the device's
local storage is U-Boot itself. Kernel, DTB, optional uInitrd and PXE
config come from TFTP; rootfs is mounted over NFS.

A new case branch in do_main_configuration auto-enables the netboot
extension, symmetric with existing fs-f2fs-support / fs-btrfs wiring.
The legacy ROOTFS_TYPE=nfs (hybrid: kernel on local storage, only /
over NFS) is untouched — both paths coexist.

- nfs-root case branch in ROOTFS_TYPE dispatch calls
  enable_extension "netboot"
- prepare_partitions skips root partition creation and SD-size sanity
  check
- check_filesystem_compatibility_on_host skipped for nfs-root

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: iav

lib/functions/configuration/main-config.sh

@@ -138,6 +138,13 @@ function do_main_configuration() {
 		nfs)
 			FIXED_IMAGE_SIZE=256 # small SD card with kernel, boot script and .dtb/.bin files
 			;;
+		nfs-root)
+			# Full netboot: no local storage in the early boot path at all. Kernel,
+			# DTB and extlinux go to TFTP, rootfs is mounted over NFS. The netboot
+			# extension owns artifact staging and PXE config generation.
+			FIXED_IMAGE_SIZE=256
+			enable_extension "netboot"
+			;;
 		f2fs)
 			enable_extension "fs-f2fs-support"
 			# Fixed image size is in 1M dd blocks (MiB)
@@ -161,9 +168,9 @@ function do_main_configuration() {
 	esac
 
 	# Check if the filesystem type is supported by the build host.
-	# Skipped for nfs: ROOTFS_TYPE=nfs produces a rootfs tarball;
-	# host-side filesystem support is irrelevant for the nfs case.
-	if [[ $CONFIG_DEFS_ONLY != yes && $ROOTFS_TYPE != nfs ]]; then
+	# Skipped for nfs/nfs-root: those produce a rootfs tarball and/or export tree;
+	# host-side filesystem support is irrelevant.
+	if [[ $CONFIG_DEFS_ONLY != yes && $ROOTFS_TYPE != nfs && $ROOTFS_TYPE != nfs-root ]]; then
 		check_filesystem_compatibility_on_host
 	fi
 

lib/functions/image/partitioning.sh

@@ -20,7 +20,7 @@ function prepare_partitions() {
 
 	# possible partition combinations
 	# /boot: none, ext4, ext2, fat (BOOTFS_TYPE)
-	# root: ext4, btrfs, f2fs, nilfs2, nfs (ROOTFS_TYPE)
+	# root: ext4, btrfs, f2fs, nilfs2, nfs, nfs-root (ROOTFS_TYPE)
 
 	# declare makes local variables by default if used inside a function
 	# NOTE: mountopts string should always start with comma if not empty
@@ -121,7 +121,7 @@ function prepare_partitions() {
 		BOOTSIZE=0
 	fi
 	# Check if we need root partition
-	[[ $ROOTFS_TYPE != nfs ]] &&
+	[[ $ROOTFS_TYPE != nfs && $ROOTFS_TYPE != nfs-root ]] &&
 		local rootpart=$((next++))
 
 	display_alert "calculated rootpart" "rootpart: ${rootpart}" "debug"
@@ -144,7 +144,7 @@ function prepare_partitions() {
 		display_alert "Using user-defined image size" "$FIXED_IMAGE_SIZE MiB" "info"
 		sdsize=$FIXED_IMAGE_SIZE
 		# basic sanity check
-		if [[ $ROOTFS_TYPE != nfs && $ROOTFS_TYPE != btrfs && $sdsize -lt $rootfs_size ]]; then
+		if [[ $ROOTFS_TYPE != nfs && $ROOTFS_TYPE != nfs-root && $ROOTFS_TYPE != btrfs && $sdsize -lt $rootfs_size ]]; then
 			exit_with_error "User defined image size is too small" "$sdsize <= $rootfs_size"
 		fi
 	else