remount: Detect when flashall has happened in the bootloader.

This adds a new metadata header flag to the super partition. This flag
is set when "adb remount" is used, and is implicitly cleared when
flashing.

If there is a scratch partition present on /data, we require that the
flag be set in order to proceed using overlays. If not set, scratch is
not mapped in first-stage init, and scratch images are removed later
during startup.

Bug: 297923468
Test: adb remount -R, touch file in out/, sync, flashall
Change-Id: I9cc411a1632101b5fc043193b38db8ffb9c20e7f