meta-agl-bsp: move usbaudio.cfg to linux-agl.inc

meta-agl-bsp: move usbaudio.cfg to linux-agl.inc

USB audio should be enabled for all AGL BSPs. Move the
usbaudio.cfg from the Renesas BSP bbappend (it's actually
not even used there as the fragment is not being appended)
to linux-agl.inc and append it.

Bug-AGL: SPEC-1148

Change-Id: I219d0d0355a4d57f883086631e07f3d191be2975
Signed-off-by: Matt Porter <mporter@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/12253
Reviewed-by: Thomas Rini <trini@konsulko.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Stéphane Desneux <stephane.desneux@iot.bzh>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>