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

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).

Signed-off-by: KaDiWa <kalle.wachsmuth@gmail.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>