kconfig: Fix pylint violations

kconfig: Fix pylint violations

Fix the following:
R0205: Class 'Config' inherits from object, can be safely removed from bases in python3 (useless-object-inheritance)
E1101: Instance of 'Config' has no '_parse' member (no-member)
W0613: Unused argument 'arch' (unused-argument)
W0613: Unused argument 'flavour' (unused-argument)
W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
R0201: Method could be a function (no-self-use)
E1101: Instance of 'Config' has no 'config' member (no-member)
W0707: Consider explicitly re-raising using the 'from' keyword (raise-missing-from)
W0105: String statement has no effect (pointless-string-statement)
W0123: Use of eval (eval-used)
W0102: Dangerous default value [] as argument (dangerous-default-value)
R1723: Unnecessary "elif" after "break" (no-else-break)
R1705: Unnecessary "elif" after "return" (no-else-return)
R1704: Redefining argument with the local name 'arch' (redefined-argument-from-local)

Signed-off-by: Juerg Haefliger <juergh@proton.me>