mdtp: porting mdtp to msm8996

Adds defines to project makefile and exports target_volume_up function
for mdtp usage.

Change-Id: Idae3899fc52b3223ea88dc0cf71a79d69c0bf10a
diff --git a/target/msm8996/init.c b/target/msm8996/init.c
index f3c1327..37d58c3 100644
--- a/target/msm8996/init.c
+++ b/target/msm8996/init.c
@@ -92,7 +92,7 @@
 }
 
 /* Return 1 if vol_up pressed */
-static int target_volume_up()
+int target_volume_up()
 {
 	uint8_t status = 0;
 	struct pm8x41_gpio gpio;