Validate that all required gcc switches were used
Validate that all required gcc switches were used

This is conservative right now and only opportunistically checks if
it can find the elf file, which I think is the right behavoir, as it
supports concepts like passing binaries around for others to flash
without getting in the way, but will likely protect users in the common
case.

We could add a warning print or s/t if the .elf isn't found, but I'm
not sure that passes muster for SNR.

helena-project/tockloader