scripts: Remove unused variables in all Python scripts

Discovered with pylint3.

Use the placeholder name '_' for unproblematic unused variables. It's
what I'm used to, and pylint knows not to flag it.

Python tip:

    for i in range(n):
        some_list.append(0)

can be replaced with

    some_list += n*[0]

Similarly, 3*'\t' gives '\t\t\t'.

(Relevant here because pylint flagged the loop index as unused.)

To do integer division in Python 3, use // instead of /.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>

scripts/mergehex.py

@@ -26,7 +26,7 @@ def merge_hex_files(output, input_hex_files):
 
         try:
             ih.merge(to_merge)
-        except AddressOverlapError as e:
+        except AddressOverlapError:
             raise AddressOverlapError("{} has merge issues".format(hex_file_path))
 
         print("Merged {}".format(hex_file_path))