improvement for emma filter option

the original implementation directly embeds the filter variable into command
line; this implies that filter variable cannot contain any whitespaces,
otherwise it'll be misintepreted as a new command line argument to emma. the
side effect is that user will be forced to cram all filter related java
class/package names into one line, which could be very long. with this
change, user can use line continuation ("\") or appending ("+=") to break
up long filter specs.

Change-Id: I2c130033cdb5120251d6f84c3dd906d140d52991

core/definitions.mk

@@ -1327,7 +1327,7 @@ endef
 define transform-classes.jar-to-emma
 $(hide) java -classpath $(EMMA_JAR) emma instr -outmode fullcopy -outfile \
     $(PRIVATE_EMMA_COVERAGE_FILE) -ip $< -d $(PRIVATE_EMMA_INTERMEDIATES_DIR) \
-    -ix $(PRIVATE_EMMA_COVERAGE_FILTER)
+    $(addprefix -ix , $(PRIVATE_EMMA_COVERAGE_FILTER))
 endef
 
 #TODO: use a smaller -Xmx value for most libraries;