base-files: /etc/banner: Adapt after image naming change

Use CONFIG_VERSION_DIST to write IOWRT version. This way, it will also
be the right text for downstream forks of our customers, who use
CONFIG_VERSION_CODE for their version number. Hardcode OpenWrt to print
the OpenWrt version, because we modify CONFIG_VERSION_DIST to be IOWRT
since feed/iopsys commit a37729d885f4 (iop: config: Set version
config-settings, 2022-09-19).

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>