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