meta-agl: Init SDL feature

meta-agl: Init SDL feature

Add new meta layer meta-agl-sdl for SmartDeviceLink feature.

Tested build ok with:
source meta-agl/scripts/aglsetup.sh -m raspberrypi3 agl-demo agl-netboot agl-appfw-smack agl-sdl

The SmartDeviceLink packages are selected by "agl-sdl" in DISTRO_FEATURES.

Bug-AGL: SPEC-805

Change-Id: I416d8418c0ef4fe4de5e5ce084e7b9075e64f85f
Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10625
Reviewed-by: Jeremiah Foster <jeremiah.foster@pelagicore.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Martin Kelly <mkelly@xevo.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>