flash: Set defaults to all unbound vars

# Common flash function to be called from device flash script

flash_factory() {

  # Begin with some OS checks and variable definition

  USE_FASTBOOTD="${USE_FASTBOOTD:-false}"

  FW_BL_RESTART="${FW_BL_RESTART:-false}"

  CLEAN_FLASH="${CLEAN_FLASH:-false}"

  VIRTUAL_AB="${VIRTUAL_AB:-false}"

  FLASH_AB_FW_IMGS="${FLASH_AB_FW_IMGS:-}"

  FLASH_A_FW_IMGS="${FLASH_A_FW_IMGS:-}"

  FLASH_AB_IMGS="${FLASH_AB_IMGS:-}"

  FLASH_A_IMGS="${FLASH_A_IMGS:-}"

  ERASE_IMGS="${ERASE_IMGS:-}"

  FORMAT_IMGS="${FORMAT_IMGS:-}"

  os_checks

  # Call function to look for device(s)