CONFIG_SERIAL_EARLYCON (not configurable)

modulename: earlycon.ko

configname: CONFIG_SERIAL_EARLYCON

Linux Kernel Configuration
└─>Device Drivers
└─>Character devices
└─>Serial drivers
└─>SERIAL_EARLYCON
In linux kernel since version 3.10 (release Date: 2013-06-30)  
Support for early consoles with the earlycon parameter. This enables
the console before standard serial driver is probed. The console is
enabled when early_param is processed.

source code:
drivers/tty/serial/earlycon.c

is selected by
CONFIG_EFI_EARLYCON