It is merely there to produce the prompt which people asked for.
CONFIG_ATA_SD (or CONFIG_ATA_BLK_DEV_SD or whatever) won't turn up in
any Makefile or source code.
Note, I'm not fond of 'select' nor of dummy Kconfig variables. Plus I
can personally live very well with the current solution (sd_mod et al
are mentioned in the help text at CONFIG_ATA). That's why I posted only
the example instead of a complete patch.
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