FEL

FEL is a low-level subroutine containing in the BootROM on allwinner devices. It is used for provisioning and recovery of devices.

There are several ways to enter it:
 * 1) Short the BSP/UBOOT pin while booting (usually brought out to volume button or similar)
 * 2) Booting a specially prepared MicroSD Image
 * 3) If BootROM detects any error, it automatically enters FEL.