BUILD: makefile: add an EXTRA_OBJS variable to help build optional code

This variable will be useful to build experimental autonomous code like
new muxes without having to patch the makefile.
diff --git a/Makefile b/Makefile
index cc29b1a..6e77b07 100644
--- a/Makefile
+++ b/Makefile
@@ -489,6 +489,9 @@
 OPTIONS_LDFLAGS =
 OPTIONS_OBJS    =
 
+#### Extra objects to be built and integrated (used only for development)
+EXTRA_OBJS =
+
 # This variable collects all USE_* values except those set to "implicit". This
 # is used to report a list of all flags which were used to build this version.
 # Do not assign anything to it.
@@ -958,6 +961,9 @@
 OBJS += src/trace.o
 endif
 
+ifneq ($(EXTRA_OBJS),)
+OBJS += $(EXTRA_OBJS)
+endif
 
 # Not used right now
 LIB_EBTREE = $(EBTREE_DIR)/libebtree.a