CONFIG_ATARI_MFPSER is not available for thedefaultarchitecture x86.
Result is shown for architecture m68k
default or selected kernelversion does not have config value CONFIG_ATARI_MFPSER.
Result is shown for kernelversion 6.2.5

Atari MFP serial support


Linux Kernel Configuration
└─>Character devices
└─>Atari MFP serial support
In linux kernel since version 2.6.12  
If you like to use the MFP serial ports ("Modem1", "Serial1") under
Linux, say Y. The driver equally supports all kinds of MFP serial
ports and automatically detects whether Serial1 is available.

To compile this driver as a module, choose M here.

Note for Falcon users: You also have an MFP port, it's just not
wired to the outside... But you could use the port under Linux.