This works (i.e., lcd.c gets compiled)  but contains a clear cut-and-paste error.
obj-$(CONFIG_STEPGEN) += stepgen.o
stepgen-objs := hal/components/stepgen.o $(MATHSTUB)
obj-$(CONFIG_STEPGEN) += lcd.o
lcd-objs := hal/components/lcd.o $(MATHSTUB)

This doesn't work, lcd.c gets ignored:
obj-$(CONFIG_STEPGEN) += stepgen.o
stepgen-objs := hal/components/stepgen.o $(MATHSTUB)
obj-$(CONFIG_LCD) += lcd.o
lcd-objs := hal/components/lcd.o $(MATHSTUB)