project: mdm9607: Define the Signed_Kernel flag

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

Change-Id: I7697b9a5e9ba850293dd0ee0b8c54453b16ed53c
diff --git a/project/mdm9607.mk b/project/mdm9607.mk
index b8ef8de..b240b32 100644
--- a/project/mdm9607.mk
+++ b/project/mdm9607.mk
@@ -12,6 +12,10 @@
 DEBUG := 1
 endif
 
+ifeq ($(TARGET_BOOTIMG_SIGNED),true)
+CFLAGS += -D_SIGNED_KERNEL=1
+endif
+
 DEFINES += WITH_DEBUG_UART=1
 DEFINES += WITH_DEBUG_LOG_BUF=1
 DEFINES += DEVICE_TREE=1