config: BoardConfigReservedSize: Prevent overlayfs on remount

* When we remount partitions via `adb remount`, a scratch partition
  is made for overlayfs to use - this takes up space and breaks OTA.
  The end user /could/ run this to fix it:
  `adb reboot fastboot && fastboot delete-logical-partition scratch && fastboot reboot`
  But that's bad UX, remount should work. This ensures that with or without GMS,
  we reserve some space on all dynamic partitions.
* Set amount to `8 * 1024 * 1024` + 1 for good measure.
* Don't reserve this amount of `-user` builds as we don't remount
  there.

Change-Id: I969aea92747a17ceca299e109c6e0ab563951374