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
In linux kernel since version 2.6.16 (release Date: 2006-03-19)  
source code:
arch/powerpc/kernel/