Makefile: add target to generate cpio

It is convenient to deliver the bootrr as a cpio archive which can be
appended to an initramfs image.

The output cpio filename can be overriding the CPIONAME variable, it
is set to 'bootrr' by default.

Also use a default value for DESTDIR, so that "make cpio" just works.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Makefile

@@ -4,6 +4,9 @@ all:
 
 all-install :=
 
+DESTDIR ?= dest
+CPIONAME ?= bootrr
+
 HELPERS := assert_file_is_empty \
            assert_device_present \
            assert_driver_present \
@@ -41,4 +44,17 @@ $(foreach v,${HELPERS},$(eval $(call add-scripts,helpers,$v)))
 
 install: $(all-install)
 
+CPIO := $(PWD)/$(CPIONAME).cpio
+
+$(CPIO): $(all-install)
+	@cd $(DESTDIR) && find ./$(prefix)/bin | cpio -o -H newc > $(CPIO)
+
+%.cpio.gz: %.cpio
+	@gzip < $< > $@
+
+cpio: $(CPIO)
+
+cpio.gz: $(CPIO).gz
+
 clean:
+	@rm -f $(CPIO) $(CPIO).gz