The nerves_system_rpi3 v1.6.3 release promises Erlang 21.2.6.
I just now updated my small project and certainly got a newer Linux kernel (Mon Feb 25) but
runtime_info shows
Elixir version: 1.8.1
Erlang/OTP version: 21
ERTS version: 10.2.4
Have I done something silly ?
nope, seems correct see https://github.com/erlang/otp/releases/tag/OTP-21.2.6 which has the corresponding erts version…
Clear and useful help ! Thanks.
This also confirms 
iex(fb2@fb2-5116)1> Application.started_applications
[
. . .
{:stdlib, 'ERTS CXC 138 10', '3.7.1'}, <=<=
{:kernel, 'ERTS CXC 138 10', '6.2'}
]
well actually erts isn’t necessarily updated for each release - see the https://github.com/erlang/otp/releases/tag/OTP-21.2.5 release where it’s only “inets” being updated…
so left as an exercise how to get the full version number of a given runtime:/