base-file: make the append work without harden distro

The append in the meta-security layer works for a "harden" distro
only. We remove this limit as we do want it by default and we do not
want to rename our distro.

A discussion on the mailing list [1] mentions that we might get
DISTRO_FEATURE in the future.

[1] https://patchwork.openembedded.org/patch/174773/



Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>