Clear vars in drivers makefile

drivers.mak is shared between hostapd and wpa_supplicant and
compiling them together may create "wrong" flags.

Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>

src/drivers/drivers.mak

@@ -1,3 +1,15 @@
+##### CLEAR VARS
+
+DRV_CFLAGS =
+DRV_WPA_CFLAGS =
+DRV_AP_CFLAGS =
+DRV_OBJS =
+DRV_WPA_OBJS =
+DRV_AP_OBJS =
+DRV_LIBS =
+DRV_WPA_LIBS =
+DRV_AP_LIBS =
+
 ##### COMMON DRIVERS
 
 ifdef CONFIG_DRIVER_HOSTAP