$ grep CONFIG_SCSI_WAIT_SCAN .config
# CONFIG_SCSI_WAIT_SCAN is not set
$ make oldconfig
$ grep CONFIG_SCSI_WAIT_SCAN .config
CONFIG_SCSI_WAIT_SCAN=m

Hm, but what does it all mean? This option does not even show up in menuconfig. This has been discussed numerous times and on LKML as well. The short story is:

The wait scan module is designed to wait for scans of driver modules.
Whether SCSI=y or m has no effect on this ... you can still have modular
drivers with built in SCSI.

And since we don't know if there are any (out-of-tree) SCSI driver modules around, we just build this module. Except when CONFIG_MODULE=n is set. But I still fail to understand why we need a .config option, when it's not (de)selectable anyway? But then again I may not fully comprehend the module-vs-builtin paradigm and SCSI_WAIT_SCAN behaves differently when builtin?