src/mainboard/google/rex/variants/screebo/Makefile.inc - coreboot - Gitiles

 bootblock-y += gpio.c

 romstage-y += gpio.c
 romstage-y += memory.c
 ramstage-y += gpio.c
	bootblock-y += gpio.c

	romstage-y += gpio.c
	romstage-y += memory.c
	ramstage-y += gpio.c