distro-build-manifest: fix incorrect DIST_LAYERS

distro-build-manifest: fix incorrect DIST_LAYERS

Using repo tool, a .repo subfolder is created beside the cloned git repositories.
It contains also some git repositories (handled by repo tool).
But when enumerating the layers, those extra ".repo/xxxx" git repositories must
be ignored to get a consistent signature on layers.

This patch excludes hidden folders (starting by '.') from the search.

Bug-AGL: SPEC-2648

Change-Id: Ibee41d4efff0898d1bf67ab661ccdf70729b6211
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>