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

CONFIG_SYS_SUPPORTS_ZBOOT_UART_PROM (not configurable)

configname: CONFIG_SYS_SUPPORTS_ZBOOT_UART_PROM

Linux Kernel Configuration
└─>CPU selection
└─>SYS_SUPPORTS_ZBOOT_UART_PROM
In linux kernel since version 4.1 (release Date: 2015-06-21)