util/broadcom/secimage: Add distclean target

When running 'make distclean' on coreboot, the build cleans the tools
as well. Since secimage didn't have a distclean target, it gave an error
that the distclean target didn't exist.  This didn't actually affect
anything more than the secimage clean, but it was impossible to tell
that from the warning:

% make distclean
make[1]: *** No rule to make target 'distclean'.  Stop.

Change-Id: I4b4bcc1ab48e767218d31e455d23527acedf4953
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/23666
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/broadcom/secimage/Makefile

@@ -36,7 +36,7 @@ test: $(TARGET)
 	@find test -maxdepth 1 -type f -executable \
 		| xargs -I "{}" sh -c "{} $$(realpath $<)"
 
-.PHONY: clean
+.PHONY: clean distclean
 
-clean:
+clean distclean:
 	$(RM) -f $(TARGET) $(OBJS)