mosquitto: bump to 1.4-development branch

This is an unreleased preview image.

libwebsockets support is included and enabled by default.  You will need
to build the package yourself to disable this, it's not considered a
variant useful enough to have it's own binaries built.

Note that at present libwebsockets always comes with openssl support, so
if you really don't want web sockets support, install the -nossl
variant.
mosquitto: Skip building documentation by default
mosquitto: allow building on older trees

mosquitto > 1.3 (1.4 dev branches and later) have support for elliptic
curve encryption, but AA and older openwrt branches don't have an ssl
library supporting that.  Make sure that EC is disabled on older builds.
mosquitto: overhaul uci conversion script

Support more options coming with mosquitto 1.4
Use more built in functions to make script simpler and easier to extend