util/broadcom/secimage: Add HMAC test

One of responsibilities of the `secimage` tool is signing the image
using the HMAC-SHA256 algorithm. The test being added verifies that
secimage's internal call yields same result as the according openssl
tool does.

Change-Id: I8de4328f435af56901a861e3d5e733657c3c7f78
Signed-off-by: Alex Thiessen <alex.thiessen.de+coreboot@gmail.com>
Reviewed-on: https://review.coreboot.org/23474
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/broadcom/secimage/Makefile

@@ -31,6 +31,11 @@ install:
 	install -d $(DESTDIR)/usr/bin
 	install $(TARGET) $(DESTDIR)/usr/bin
 
+.PHONY: test
+test: $(TARGET)
+	@find test -maxdepth 1 -type f -executable \
+		| xargs -I "{}" sh -c "{} $$(realpath $<)"
+
 .PHONY: clean
 
 clean: