this adds support for devices with cellular modems, like the GL-inet
XE300

Some devices (e.g. TP-Link RE450) only have 6 MB available space. That's
not enough to hold all the stuff e.g. ca-certs. So only build
ca-certificates, airtime stats etc. when we have enough flash.

when running opkg update, this currently results in errors like this:

Downloading https://downloads.openwrt.org/releases/22.03-SNAPSHOT/packages/arm_cortex-a7_neon-vfpv4/base/Packages.gz
wget: SSL support not available, please install one of the libustream-.*[ssl|tls] packages as well as the ca-bundle and ca-certificates packages.
*** Failed to download the package list from https://downloads.openwrt.org/releases/22.03-SNAPSHOT/packages/arm_cortex-a7_neon-vfpv4/base/Packages.gz

since we already have wolfssl for hostapd stuff, just install
libustream-wolfssl and ca-certificates.

Build fails with it, let's see if we need it.

As per Gluon recommendation, remove haveged from SITE_PACKAGES:
> Starting with version 19.07 OpenWrt ships the urngd entropy daemon by
> default. It replaces the haveged daemon, for which we removed the
> support in Gluon. Remove haveged from your package selection.

Reference: https://gluon.readthedocs.io/en/latest/releases/v2020.2.html#site-mk

It is now possible to use usb network cards with a x86 offloader like a
futro, removing the need for a pcie network card

Citing the gluon v2018.1 release notes:
    This version changes the flash partition layout on some devices (TP-Link
    CPE/WBS 210/510). To avoid upgrade failures, make sure to upgrade to
    Gluon 2017.1.8 or the latest Gluon 2016.2.x (unreleased) before
    installing Gluon 2018.1.

So we need a yet unreleased ffs-1.4 version and have to bump the version
to 1.5 fot a release with gluon v2018.1.

Setting BUILD_DATE overrides the date used in the generated image names.
With this it is possible to have a consistent build date across all
architectures, making it easier to identify a release. Furthre more it
might be the key to have reproducible builds

gluon-web-mesh-vpn-fastd allows the user to switch between the security and performance VPN settions.