staging: Introduce /sysroot-only to SYSROOT_DIRS

staging: Introduce /sysroot-only to SYSROOT_DIRS

The '/sysroot-only' in SYSROOT_DIRS is to be used by recipes which
generate artifacts that are not included in the target filesystem.

Also, remove the ${D}/sysroot-only dir before copying D do PKGD to
generate the packages since it is not supposed to be included in
any package.

This will allow recipes to share non-target filesystem artifacts
without needing to use the DEPLOY_DIR and keep it tidy.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>