CONFIG_SYS_SUPPORTS_ZBOOT_UART16550 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_SYS_SUPPORTS_ZBOOT_UART16550 (not configurable)

configname: CONFIG_SYS_SUPPORTS_ZBOOT_UART16550

Linux Kernel Configuration
└─>CPU selection
└─>SYS_SUPPORTS_ZBOOT_UART16550
In linux kernel since version 2.6.33 (release Date: 2010-02-24)