main/postmarketos-mkinitfs: Stop forcing full fsck on every boot

At the moment, a full file system check is forced even if the file system is marked clean (i.e. it was properly unmounted the last time it was used). On large file systems with slow I/O (e.g. SD cards), this can take a significant amount of time.

Removing the -f option of e2fsck allows skipping the full file system check if it is marked clean, speeding up the boot time.

On my me176c with my 128 GB SD card, the full file system check delays the boot for like 3 minutes or so, whereas it boots instantly with this patch. (Actually it's a pretty fast SD card, the SD card reader in me176c is the limitation...)