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>