Allow battery thresholds to be set in config.mk

The start/end thresholds are user-configurable values. This will allow
us to override these with profile values if desired.

src/board/system76/common/common.mk

@@ -25,9 +25,12 @@ INCLUDE+=$(wildcard $(SYSTEM76_COMMON_DIR)/include/common/*.h) $(SYSTEM76_COMMON
 CFLAGS+=-I$(SYSTEM76_COMMON_DIR)/include
 
 # Set battery charging thresholds
+BATTERY_START_THRESHOLD?=0
+BATTERY_END_THRESHOLD?=100
+
 CFLAGS+=\
-	-DBATTERY_START_THRESHOLD=0 \
-	-DBATTERY_END_THRESHOLD=100
+	-DBATTERY_START_THRESHOLD=$(BATTERY_START_THRESHOLD) \
+	-DBATTERY_END_THRESHOLD=$(BATTERY_END_THRESHOLD)
 
 # Add charger
 CHARGER?=bq24780s