checkkconfigsymbols.py: find similar symbols

Add support to find string-similar symbols.  When option --sim SYM is
specified, checkkconfigsymbols.py will print at most 10 symbols defined
in Kconfig that are string similar to SYM in the following format:
    Similar symbols: $COMMA_SEPARATED_LIST_OF_SYMBOLS

Note, if no similar symbols are found it is indicated as follows:
    Similar symbols: no similar symbols found

Since the implemented functionality is also useful when searching the
entire source or when diffing two commits, a list of similar symbols is
printed unconditionally with the other data.  In order to make the
output more readable, the format now looks as follows:

    $UNDEFINED_SYMBOL
    Referencing files: $COMMA_SEPARATED_LIST_OF_FILES
    Similar symbols: $COMMA_SEPARATED_LIST_OF_SYMBOLS
    [Optional with '--find']
    Commits changing symbol:
        - $COMMIT_1_HASH ("$COMMIT_1_MESSAGE")
        - $COMMIT_2_HASH ("$COMMIT_2_MESSAGE")
            or
        - no commit found

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>