scripts: footprint: Fix pylint warnings

 - Remove unused variables and an unused 'sys' import

 - Simplify 'if len(foo) != 0' to 'if foo'. Non-empty lists/dicts/etc.
   are truthy in Python.

 - Use a raw string to fix this warning:

       scripts/footprint/size_report:270:0: W1401: Anomalous backslash
       in string: '\.'. String constant might be missing an r prefix.
       (anomalous-backslash-in-string)

   The issue is that '\.' just happens to work due to not being
   recognized as an escape sequence.

   Escape sequences are not interpreted in raw strings, so they're safer
   for regexes.

 - Replace 'is 0' with '== 0'. 'is' is for testing object identity, and
   0 isn't guaranteed to be a unique object (but always is in practice).

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