Remove dependency to autoconf-archive

It is generally not a good idea to add more and more external
dependencies unless absolutely needed.  autoconf-archive is a good
example of that, as we need to update all build machines, and older
OpenEmbedded versions do not appear to have (the right?)
autoconf-archive recipe. Provide local copy of necessary m4 files to fix
the build there.

The dependency to autoconf-archive was introduced in Change-Id
Ied9c950dafa65f324cf31298b13b590f56139700

Change-Id: Iacc1958b471ec3fc65307259039e1d496845f528
diff --git a/openbsc/m4/README b/openbsc/m4/README
new file mode 100644
index 0000000..92eb30b
--- /dev/null
+++ b/openbsc/m4/README
@@ -0,0 +1,3 @@
+We want to avoid creating too many external build-time dependencies
+like this one to autoconf-archive.  This directory provides a local
+copy of required m4 rules.