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>

[media] Fix some Makefile rules

On a few places, := were using instead of +=, causing drivers to
not compile.

While here, standardize the usage of += on all cases where multiple
lines are needed, and for obj-y/obj-m targets, and := when just one
line is needed, on <module>-obj rules.

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Identified-by: Antti Polosaari <crope@iki.fi>
Tested-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

[media] rename most media/video usb drivers to media/usb

Rename all USB drivers with their own directory under
drivers/media/video into drivers/media/usb and update the
building system.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>