README: Adapt the README for SECO

Rename the distribution to SECO Northern Europe distribution and
reference the manifest as the main source of information.

BCS 746-000659

(cherry picked from commit 81bd01a9)

README.md

 # meta-seconorth-distro
+
 See README.md in the manifest project for project information.
 
 This layer contains everything to build the standard SECO Northern Europe distribution,  containing all tools and packages generally used on SECO Northern Europe (by Garz & Fricke) devices.