project: mdm9640: Define the Signed_Kernel flag

Signed Kernel flag should be defined based on the
TARGET_BOOTIMG_SIGNED flag in bootimage compilation.

Change-Id: I0fa48e1fdc80953f472507c231e4f30e3bc4ea3c
diff --git a/project/mdm9640.mk b/project/mdm9640.mk
index 2aa002a..2c81541 100644
--- a/project/mdm9640.mk
+++ b/project/mdm9640.mk
@@ -10,6 +10,10 @@
 DEBUG := 1
 endif
 
+ifeq ($(TARGET_BOOTIMG_SIGNED),true)
+  CFLAGS += -D_SIGNED_KERNEL=1
+endif
+
 ENABLE_USB30_SUPPORT := 1
 ENABLE_SDHCI_SUPPORT := 1
 ENABLE_BOOT_CONFIG_SUPPORT := 1