Clarify recovery_exit docs and investigate the use of PowerOn()
In the recovery mode support, the recovery_exit unconditionally adds the PowerOn() action to the pipeline. This is probably wrong in all cases and certainly causes issues with the X15-bl as the next action will do a reboot anyway but the prompts and device output will already be in the ser2net buffer. Clarify the documentation that recovery_exit does not have to be used but exists in case the recovery_exit commands in the device dictionary need to differ from the power_off commands in the device dictionary for some reason or if an explicit exit is required.
Once https://lava.codehelp.co.uk/scheduler/device/banjo is available again as a hikey U-Boot device, use https://validation.linaro.org/scheduler/device_type/hi6220-hikey-bl to recover into U-Boot and test.