Fix empty boot_out.txt
Fix empty boot_out.txt

A pointer to the FIL structure was kept after it went out of scope.
Apparently this happened to work until cf97793 added the `result`
variable that clobbered it.

Fixes #5062

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython