CONFIG_DEBUG_LL_UART_EFM32 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging via efm32 UART

configname: CONFIG_DEBUG_LL_UART_EFM32

Kernel low-level debugging port
└─>Kernel low-level debugging via efm32 UART
In linux kernel since version 3.13 (release Date: 2014-01-19)  
Say Y here if you want the debug print routines to direct
their output to an UART or USART port on efm32 based
machines. Use the following addresses for DEBUG_UART_PHYS:

0x4000c000 | USART0
0x4000c400 | USART1
0x4000c800 | USART2
0x4000e000 | UART0
0x4000e400 | UART1