distro-manifest-generator: enhance git repos detection

distro-manifest-generator: enhance git repos detection

Following recent change 334a201 (#21619), it's now necessary to
explore the layers folder to find git repositories.

But when the build directory is a subfolder of $METADIR (repo folder),
'find' digs too deeply to find git repositories.

This patch adds some limits for 'find' to dig into subfolders:
* only find "top" git repos (no git submodules)
* limit the search depth to 2

Bug-AGL: SPEC-2450

Change-Id: Idd907737673d196cc8ba157a5c49492119efeb88
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>