makefile: parse and process flags to enable ffbm mode on systemd for LE

Pass a flag to make utility to ensure "systemd.target" kernel
commandline parameter is added when systemd is
used as the init manager on LE platform. This is
done by reading flags sent by EXTRA_OEMAKE in the
lk recipe file.

Change-Id: Ic9330c260fd03b25122c90a1c97bcdec48b14570
diff --git a/makefile b/makefile
index 7082311..b6c1ddb 100644
--- a/makefile
+++ b/makefile
@@ -121,6 +121,12 @@
   DEFINES += USER_BUILD_VARIANT=1
 endif
 
+ifeq ($(USE_LE_SYSTEMD),true)
+  DEFINES += USE_LE_SYSTEMD=1
+else
+  DEFINES += USE_LE_SYSTEMD=0
+endif
+
 # these need to be filled out by the project/target/platform rules.mk files
 TARGET :=
 PLATFORM :=