genrandconfig: fix (some) pep8 warnings

Warnings fixed:
E731 do not assign a lambda expression, use a def
 -> urlopen_closing is defined with a def. urlopen is not used
    elsewhere so inlined.
E302 expected 2 blank lines
E501 line too long
 -> long lines due to a long string are NOT split
E701 multiple statements on one line (colon)
E722 do not use bare except'
 -> use "except Exception", so KeyInterrupt and SystemExit are still
    passed. We never intended to catch those.
E741 ambiguous variable name 'l'
 -> variable name is replaced with the much more descriptive
    toolchains_csv
E271 multiple spaces after keyword
E231 missing whitespace after ','

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>