libcmds: Write more bblayers.conf boilerplate settings

Irrespective of build system (OE / Yocto / Isar), they all have

    BBPATH = "${TOPDIR}"
    BBFILES ?= ""

in their bblayers_conf_header fragments. This could be dropped if kas
wrote those standard assignments already. So let's do that, weakly so
that those could still be overwritten via bblayers_conf_header.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/libcmds.py

@@ -256,6 +256,8 @@ class WriteBBConfig(Command):
                     sorted(layer for repo in ctx.config.get_repos()
                            for layer in repo.layers)))
                 fds.write('"\n')
+                fds.write('BBPATH ?= "${TOPDIR}"\n')
+                fds.write('BBFILES ??= ""\n')
 
         def _write_local_conf(ctx):
             filename = ctx.build_dir + '/conf/local.conf'