Add DOXYGEN definition, comment out reference to not-yet-existent "image"

directory.

src/Makefile

 SYMCHECK	?= $(PERL) ./util/symcheck.pl
 SORTOBJDUMP	?= $(PERL) ./util/sortobjdump.pl
 NRV2B		?= ./util/nrv2b
+DOXYGEN		?= doxygen
 
 # Location to place generated files
 #
 #
 SRCDIRS		+= core
 SRCDIRS		+= proto
-SRCDIRS		+= image
+#SRCDIRS		+= image
 SRCDIRS		+= drivers/bus
 SRCDIRS		+= drivers/net
 #SRCDIRS		+= drivers/disk