scripts/check_complaince: Add configs to UNDEF_KCONFIG_WHITELIST

Adding LOG_BACKEND_MOCK_OUTPUT_DEFAULT &
LOG_BACKEND_MOCK_OUTPUT_SYST to UNDEF_KCONFIG_WHITELIST as Kconfig
options generated by a template under testcases are not
identified by Compliance check by default.

Signed-off-by: Aastha Grover <aastha.grover@intel.com>