treewide: Add SPDX license identifier - Makefile/Kconfig

Add SPDX license identifiers to all Make/Kconfig files which:

- Have no license information of any form

These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:

GPL-2.0-only

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

ALSA: synth: Select snd-emux-synth explicitly

Instead of the non-standard way to enable the build of snd-emux-synth
module inside Makefile, rewrite Kconfig to select the item explicitly
from each driver (sbawe and emu10k1). This is the standard way.

Signed-off-by: Takashi Iwai <tiwai@suse.de>