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>