checkkconfigsymbols.py: fix sorted output

Commit b1a3f243 ("checkkconfigsymbols.py: make it Git aware")
mistakenly removed to print undefined Kconfig symbols in alphabetical
order.  Furthermore, the script does not print anything anymore when the
entire tree is checked (i.e., when no commit is specified).

This patch restores the sorted output and adds the missing print for the
default case.  Additionally, the file lists are now sorted as well which
(a) makes it easier to read and (b) makes the output deterministic.

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