bitbake-layers: add ability to fetch layers and their dependencies from layer index

bitbake-layers: add ability to fetch layers and their dependencies from layer index

Add a command to query layer dependencies from a layer index such as the
OpenEmbedded Layer Index at http://layers.openembedded.org. Fetches the
layer and its dependencies and adds them into conf/bblayers.conf.

[YOCTO #5348]

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>