scripts/checkkconfigsymbols.py: support default statements

Until now, checkkonfigsymbols.py did not check default statements for
references on missing Kconfig symbols (i.e., undefined Kconfig options).
Hence, add support to parse and check the Kconfig default statement.

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