TT#118659 EFI support: ensure to always have a proper FAT filesystem available

If grml-debootstrap detects an existing FAT filesystem on the EFI partition,
it doesn't modify/re-create it:

| EFI partition /dev/nvme0n1p2 seems to have a FAT filesystem, not modifying.

The underlying check is execution of `fsck.vfat -bn $DEVICE`.

Now with fsck.fat from dosfstools v4.1-2 as present in Debian/buster we got:

| root@grml ~ # fsck.vfat -bn /dev/nvme0n1p2
| fsck.fat 4.1 (2017-01-24)
| 0x41: Dirty bit is set. Fs was not properly unmounted and some data may be corrupt.
|  Automatically removing dirty bit.
| There are differences between boot sector and its backup.
| This is mostly harmless. Differences: (offset:original/backup)
|   0:00/eb, 82:00/46, 83:00/41, 84:00/54, 85:00/33, 86:00/32, 87:00/20
|   , 88:00/20, 89:00/20, 510:00/55, 511:00/aa
|   Not automatically fixing this.
| Leaving filesystem unchanged.
| 1 root@grml ~ #

Now with dosfstools v4.2-1 as present in Debian/bullseye, this might become:

| root@grml ~ # fsck.vfat -bn /dev/nvme0n1p2
| fsck.fat 4.2 (2021-01-31)
| There are differences between boot sector and its backup.
| This is mostly harmless. Differences: (offset:original/backup)
|   0:00/eb, 65:01/00, 82:00/46, 83:00/41, 84:00/54, 85:00/33, 86:00/32
|   , 87:00/20, 88:00/20, 89:00/20, 510:00/55, 511:00/aa
|   Not automatically fixing this.

In such situations we end up with an incomplete/broken EFI partition,
which breaks within our efivarfs post-script:

| Mounting /dev/nvme0n1p2 on /boot/efi
| mount: /boot/efi: wrong fs type, bad option, bad superblock on /dev/nvme0n1p2, missing codepage or helper program, or other error.
|  ESC[31;01m-> Failed (rc=1)ESC[0m
|  ESC[32;01m*ESC[0m Removing chroot-script again
|  ESC[32;01m*ESC[0m Executing post-script /etc/debootstrap/post-scripts//efivarfs
| Executing /etc/debootstrap/post-scripts//efivarfs
| Mounting /dev (via bind mount)
| Mounting /boot/efi
| mount: /boot/efi: special device UUID= does not exist.

Change-Id: I46939b4e191982a84792f3aca27c6cc415dbdaf4

templates/scripts/includes/deployment.sh

@@ -2028,6 +2028,10 @@ if [[ -n "${EFI_PARTITION}" ]] ; then
     echo "EFI support present, enabling EFI support within grml-debootstrap"
     EFI_OPTION="--efi ${EFI_PARTITION}"
 
+    # ensure we force creation of a proper FAT filesystem
+    echo "Creating FAT filesystem on EFI partition ${EFI_PARTITION}"
+    mkfs.fat -F32 -n "EFI" "${EFI_PARTITION}"
+
     # this can be dropped once we have grml-debootstrap >=v0.99 available in our squashfs
     efivars_workaround
   else