Use shorter fs label for EFI partition to not break with recent dosfstools

Use shorter fs label for EFI partition to not break with recent dosfstools

This used to work fine until dosfstools 4.1:

| root@grml ~ # mkfs.fat -F32 -n "EFI System Partition" /dev/loop1
| mkfs.fat 4.1 (2017-01-24)
| mkfs.fat: warning - lowercase labels might not work properly with DOS or Windows

Now with dosfstools >=4.2 it's failing though:

| root@grml ~ # mkfs.fat -F32 -n "EFI System Partition" /dev/loop1
| mkfs.fat 4.2 (2021-01-31)
| mkfs.fat: Label can be no longer than 11 characters

So instead use "EFI" as filesystem label, to not fail with newer dosfstools.

Closes: #987014
Closes: grml/grml-debootstrap#168