Makefile: replace `echo` with `printf` for compatibility

[kadiwa4]

I don't know why but echo -n didn't work for me. macOS supports the -n option but it doesn't work in the makefile. What it does instead is it literally writes -n to the file and then also leaves a newline at the end.

I'm using GNU Make 4.4 (gmake from Homebrew).

[avpatel]

We have mailing list based patch review so it would be great if you can send these patchs to OpenSBI mailing list.

You need to join OpenSBI mailing list using following link: http://lists.infradead.org/mailman/listinfo/opensbi

Make sure you use "git send-email" to send the patches.

Regards, Anup