Interface with health HAL to read battery voltage

Bug: 78793464
Test: fastboot getvar battery-voltage
Change-Id: Ie0763e4f08327ec7649c5629066eb73e8142e0e6
diff --git a/fastboot/constants.h b/fastboot/constants.h
index c11d9de..2eaf006 100644
--- a/fastboot/constants.h
+++ b/fastboot/constants.h
@@ -62,3 +62,4 @@
 #define FB_VAR_HW_REVISION "hw-revision"
 #define FB_VAR_VARIANT "variant"
 #define FB_VAR_OFF_MODE_CHARGE_STATE "off-mode-charge"
+#define FB_VAR_BATTERY_VOLTAGE "battery-voltage"