pkg/oonf_api: Use MRI script to combine archives.

The OONF package is combining multiple ".a" file into a single archive. The
way it was being done involved creating and changing directories, unpacking
the original archives and repacking them into a combined one.

Theis has a couple of issues:

- It is untidy and wasteful.
- It breaks when thin archives are enabled, as a thin archive cannot be
  unpacked.

This commit uses a MRI script to do the combining step. It works both with
and without thin archives and is cleaner overall.

An issue that remains to be soved is that make is calling itself to create the
archive, as the PARTIAL_ARCHIVES are not known before hand. This is hacky. It
can be solved but it is a subject for another PR.

pkg/oonf_api/Makefile

@@ -8,14 +8,34 @@ MODULE:=$(PKG_NAME)
 # GCC 7.x fails on (intentional) fallthrough, thus disable implicit-fallthrough.
 CFLAGS += -Wno-implicit-fallthrough
 
+COMBINED_ARCHIVE = $(BINDIR)/$(MODULE).a
+
 .PHONY: all
 
 all:
 	"$(MAKE)" -C $(PKG_BUILDDIR)
-	"$(MAKE)" $(BINDIR)/$(MODULE).a
+	"$(MAKE)" $(COMBINED_ARCHIVE)
 
-$(BINDIR)/$(MODULE).a: $(BINDIR)/oonf_*.a
-	mkdir -p $(BINDIR)/$(MODULE); cd $(BINDIR)/$(MODULE); for var in $?; do ar -x $$var; done; ar -r -c -s $(BINDIR)/$(MODULE).a *.o
+PARTIAL_ARCHIVES = $(wildcard $(BINDIR)/oonf_*.a)
+
+$(COMBINED_ARCHIVE): $(BINDIR)/$(MODULE).mri $(PARTIAL_ARCHIVES)
+	ar -M < $<
+
+define ADDLIB_TEMPLATE
+addlib $1
+
+endef
+
+define MRI_TEMPLATE
+create $1
+$(foreach a,$2,$(call ADDLIB_TEMPLATE,$a))
+save
+end
+endef
+
+$(BINDIR)/$(MODULE).mri:
+	$(file >$@,$(call MRI_TEMPLATE,$(COMBINED_ARCHIVE),$(PARTIAL_ARCHIVES)))
+	@true
 
 include $(RIOTBASE)/pkg/pkg.mk
 ifneq (,$(filter -Wformat-nonliteral -Wformat=2, $(CFLAGS)))