CONFIG_PPC_UDBG_16550 is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc
CONFIG_PPC_UDBG_16550 (not configurable)
modulename: legacy_serial.ko
configname: CONFIG_PPC_UDBG_16550
Linux/PowerPC Kernel Configuration
└─>PPC_UDBG_16550