trezor-firmware/core/tools/combine_firmware

#!/usr/bin/env python3
import sys


def pairwise(iterable):
    a = iter(iterable)
    return zip(a, a)


files = sys.argv[1:]
files = list(pairwise(files))

offset = int(files[0][0], 16)

out = bytearray()

for addr, fn in files:
    addr = int(addr, 16) - offset
    data = open(fn, "rb").read()
    if len(out) < addr:
        out += b"\x00" * (addr - len(out))
    if len(out) != addr:
        raise Exception("Alignment failed")
    out += data

sys.stdout.buffer.write(out)