makefiles/riotboot.mk: enforce rule precedence (signed.bin before .bin)

make is a bit strange sometimes and considers various factors to decide
the precedence of the rule to be applied: definition order is one (and
the %.signed.bin is after %.bin, and would be also if we put it at the
end of Makefile.include), but also if a rule is implicit is then
preferred no matter the order.

A quite complete and coincise overview of the topic can be found here:
https://stackoverflow.com/questions/28842851/force-make-to-use-a-more-specific-rule

Given the inclusion mechanism used in RIOT it should be safer to
explicitly state things, although may seem ugly sometimes.

makefiles/riotboot.mk

@@ -34,7 +34,7 @@ $(BINDIR)/$(APPLICATION)-slot1.elf: ROM_OFFSET=$(SLOT1_OFFSET)
 $(BINDIR)/$(APPLICATION)-slot2.elf: ROM_OFFSET=$(SLOT2_OFFSET)
 
 # create signed binary target
-%.signed.bin: %.sig %.bin
+$(BINDIR)/$(APPLICATION)-slot1.signed.bin $(BINDIR)/$(APPLICATION)-slot2.signed.bin: %.signed.bin: %.sig %.bin
 	@echo "creating $@..."
 	cat $^ > $@