Enforce unified tunings across all target boards of AGL

Enforce unified tunings across all target boards of AGL

DEFAULTTUNE is a setting that should be defined in the DISTRO.
So we do it here. Goal is to have just 3-4 SDKs in the end.

We use these levels:
- ARM 32bit high:   armv7vethf-neon-vfpv4  (=default for ARM 32bit)
- ARM 32bit medium: armv7athf-neon         (enabled via DISTRO_FEATURE_append = " agl-medium-arm-compiler")
- AARCH64:          aarch64                (=aarch64, no other tunings)
- x86-64:           corei7-64              (=corei7-64 also for qemux86-64)

v2: Fix qemu user-mode emulation on x86-64 and adapt qemux86-64 tune flags.

See: https://goo.gl/DMaBJ9

Bug-AGL: SPEC-491

Change-Id: Iababbe38a531d546c03f695847651f2d83301b9a
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9299
Reviewed-by: Martin Kelly <mkelly@xevo.com>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Stéphane Desneux <stephane.desneux@iot.bzh>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>