Add missing boot.4th verification

During initialization of the forth interpreter
the loader looks for "/boot/boot.4th"
and executes any code found there.
That file was loaded bypassing verification.
Add a call to verify_file to change that.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: sjg
Obtained from: Semihalf
Sponsored by: Stormshield