postmarketos-base: post-install script assumes ext4 root filesystem

The post-install script of postmarketos-base creates an /etc/fstab with the root filesystem set as ext4, which can be seen in https://gitlab.postmarketos.org/postmarketos/pmaports/-/blob/7b7c1db0ff148fe29f8d9e4fcd087c10ad38bb2f/main/postmarketos-base/postmarketos-base.post-install#L39

In order to allow different filesystems to be chosen for the root filesystem, a mechanism needs to be introduced that would allow pmbootstrap to propagate to postmarketos-base.post-install the information of which filesystem was selected for the root fs.