resizerootfs: switch from using sfdisk to parted for resizing partitions

sfdisk is a bit crusty - it doesn't understand gpt partition tables very well,
for example. By switching to parted, we can handle gpt issues (which may be
useful in the future, and is definitely useful for other boards), and we no
longer have to hardcode that 4M alignment workaround. Parted will tell us
the free space at the end of the disk.

Because we're already using partprobe, there's no additional dependencies
needed.

rootfs/etc/initramfs-tools/hooks/rpi-resizerootfs

@@ -18,6 +18,6 @@ esac
 #copy_exec /usr/bin/tail
 copy_exec /sbin/blkid
 copy_exec /bin/lsblk
-copy_exec /sbin/sfdisk
+copy_exec /sbin/parted
 copy_exec /sbin/partprobe
 copy_exec /sbin/resize2fs

rootfs/etc/initramfs-tools/scripts/local-bottom/rpi-resizerootfs

@@ -18,9 +18,14 @@ rootpart=$(realpath $ROOT)
 rootpart_nr=$(blkid -sPART_ENTRY_NUMBER -o value -p $rootpart)
 rootdev="/dev/$(lsblk -no pkname "$rootpart")"
 
-# Check if there's free space on the device (note: we align the first
-# partition at 4MB, so there's always at least 3MB free)
-free_space="$(sfdisk -qF $rootdev | tail -n1 | grep -v [^0-9]3M)"
+# Parted will detect if the GPT label is messed up and fix it
+# automatically, we just need to tell it to do so.
+parted -s $rootdev print 2>&1 | grep -z "fix the GPT" && {
+    echo "Fix" | parted ---pretend-input-tty $rootdev print
+}
+
+# Check if there's free space at the end of the device
+free_space="$(parted -m -s $rootdev print free | tail -n1 | grep free)"
 if test -z "$free_space"; then
     # Great, we already resized; nothing left to do!
     exit 0
@@ -32,9 +37,7 @@ log_begin_msg "$0 resizing $ROOT"
 umount "${rootmnt}"
 
 # Expand the partition size to fill the entire device
-sfdisk -f $rootdev -N $rootpart_nr <<EOF
-,+
-EOF
+parted -s $rootdev resizepart $rootpart_nr 100%
 
 wait_for_udev 5