main/postmarketos-mkinitfs: iterate over all block devices and partitions in init_functions.sh:mount_subpartitions()

Previously, mount_subpartitions would only look for subpartitions of partitions whose device file path starts with /dev/mmcblk.

This seems to have caused issues on at least one device (xiaomi-gemini).

This commit makes mount_subpartitions parse the contents of /proc/diskstats in order to find all disk devices, which it then iterates over.

It might also allow people to use external storage, attached over usb, as the root fs.