linux-yocto-custom: provide defconfig guidance

linux-yocto-custom: provide defconfig guidance

As reported by Andrea, defconfigs created with savedefconfig should specify
their expansion mode as "--alldefconfig' for custom yocto recipes. To ensure
that this is documented, we can add a comment in the skeleton recipe.

Reported-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>