Support setting the flavor of an enterprise edition using compile options.
The customized EMQX_FLAVOR will be printed in the EMQX welcome message:
EMQX_FLAVOR=niceday make emqx-enterprise
./bin/emqx start
EMQX Enterprise(niceday) 5.8.3-g99ca2ea8 is started successfully!
One can get the flavor by emqx_release:get_flavor/0:
5.8.3-g99ca2ea8(emqx@127.0.0.1)1> emqx_release:get_flavor().
niceday