fix: Make blisp check for existing file before flashing

[shymega]

This commit adds a simple check to both blisp commands that checks the
passed firmware .bin file exists, and is readable.

Justification: I flashed my Pinecil V2 that arrived today, and
misspelled the filename. blisp erased flash, and then exited - it did
not check the firmware .bin was readable/accessible, which meant my
Pinecil was soft-bricked.

I have tested the change, and it works for both commands when the input
file isn't readable.

I had to declare the cmd_{iot,write}_args_print_glossary function as
static before the call, so that we don't use undeclared functions before
we call them.

Signed-off-by: Dom Rodriguez [email protected]