s/system partition/rootfs: fix remaining mentions
Follow-up to !1373 (merged), where pmbootstrap flasher flash_system
was
replaced with pmbootstrap flasher flash_rootfs
. We still had used
terms like "system partition" in a lot of places.
This commit replaces it everywhere, so it's clear that we're talking about the pmOS rootfs (which may or may not be installed to Android's system partition).