Build only 'reasonable' combinations of APP/MEMORY

There's no point in building a DFU loeader that is to be flashed
via DFU - nor is there really any need for regular cardem/trace
that can be flahsed directly without DFU.  If anyone needs those,
they can still build them - but let's not confuse the average other
user.

Change-Id: I0abe86c6a942a59e5b2417d0532dffae654d7a18
Closes: OS#4087
diff --git a/firmware/Makefile b/firmware/Makefile
index a684424..79e9a31 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -37,12 +37,14 @@
 # (can be overriden by adding CHIP=chip and BOARD=board to the command-line)
 CHIP  ?= sam3s4
 BOARD ?= qmod
+APP ?= dfu
 
 # Defines which are the available memory targets for the SAM3S-EK board.
-MEMORIES ?= flash dfu
-
-# Output file basename
-APP ?= dfu
+ifeq ($(APP), dfu)
+MEMORIES ?= flash
+else
+MEMORIES ?= dfu
+endif
 
 # Output directories and filename
 OUTPUT = $(BOARD)-$(APP)