scripts/signrom: remove Python 2 support, add shebang

#!/usr/bin/env python3

from __future__ import print_function

#

# Option ROM signing utility

checksum = 0

for b in data:

    # catch Python 2 vs. 3 differences

    if isinstance(b, int):

        checksum += b

    else:

        checksum += ord(b)

checksum = (256 - checksum) % 256

    checksum = (checksum - b) & 255

# Python 3 no longer allows chr(checksum)

fout.write(struct.pack('B', checksum))

fin.close()