scripts: ci: check_compliance: allow mcuboot log level dbg

Currently, only MCUBOOT_LOG_LEVEL_INF and MCUBOOT_LOG_LEVEL_WRN
are allowed in UNDEF_KCONFIG_ALLOWLIST. Let's allow
MCUBOOT_LOG_LEVEL_DBG as well, since some use cases may require
this level of scrutiny.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>