meta/conf/layer.conf: adapt to more flexible initramfs-framework RDEPENDS
initramfs-framework now RDEPENDS on ${VIRTUAL-RUNTIME_base-utils},
which can be busybox or some alternative like toybox. Making the
SIGGEN_EXCLUDE_SAFE_RECIPE_DEPS exception flexible, too, ensures that
distros using toybox still pass the selftests.
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>