Use the builtin RTC and display it

Its roughly %15 fast, and initializes to zero, but the display
currently shows HH:MM:SS AM/PM.

Makefile

@@ -70,7 +70,7 @@ DEVICE_DEFINE = $(subst XX,xx,$(shell echo $(DEVICE_FAMILY) | tr '[:lower:]' '[:
 # C pedantism
 CFLAGS =  -Wall -Wextra -Wpedantic
 # Debug/optimization
-CFLAGS += -Og -g3
+CFLAGS += -Og -ggdb
 # Architecture
 CFLAGS += -mthumb -mcpu=cortex-m0plus
 CFLAGS += -ffreestanding
@@ -99,7 +99,7 @@ build: $(OUTPUT_BIN)
 
 %.o: %.c
 	@echo "CC $@"
-	$(CC) $(CFLAGS) -c $< -o $@
+	@$(CC) $(CFLAGS) -c $< -o $@
 
 %.o: %.S
 	@echo "AS $@"