List files in a directory
You do not use file_list
Emptiness of container (as in PEP8) is usually checked by if not container.
For path operations os.path is preferred. Let's change 'final' to os.path.dirname and building path to os.path.join. Avoid os-specific solutions.
Let's reduce indentation by using continue statement.
IMHO using comprehensions is not preferable here. Pythonic means 'obvious' and 'simple' way, and this is very simple way.
I introduced explanatory variable and tweaked naming a little bit.
Consider separating building file list with writing it to file. Separating responsibilities is usually great idea. If you do that maybe you'll be able to find good place to use comprehensions.


Improved code (without separating) below:

import os
with open("fileList.txt", "w") as fp:
    for root, folders, files in os.walk('path/start/'):
        if folders:
            continue
        for fil in files:
            if not fil.endswith('img'):
                continue
            dir_name = os.path.basename(fil)
            full_path = os.path.join(root, fil)
            fp.write("{0} {1}\n".format(full_path, dir_name))