Draft: main/postmarketos-mkinitfs: include vfat in initfs by default

An increasing number of devices relies on the vfat file system in the initfs (e.g. for UEFI boot). Having to add all the modules required for it to work to deviceinfo_modules_initfs is annoying and inconsistent, in particular for the additionally chosen "nls" modules. Since we already bundle several file systems by default, let's just add vfat as well to simplify this.

Ideally mkinitfs should detect the file systems chosen at install time in the future and only include the necessary modules, but until then this just adds a couple of kilobytes to the initramfs.

Note: I don't have an extremely strong opinion about this, but I got a bit annoyed about the length of deviceinfo_initfs_modules when trying to make asus-me176c use linux-edge instead of the custom kernel...