It wasn't checked for `NULL`, and on CC3200 NULL dereferencing addresses
doesn't cause a crash, so it worked by pure luck: ctx->ssl_key was 0.
After `mg_` to `miot_` refactoring it's not the case anymore (presumably
because linker arranged objects in a different order), so this bug shown
up.

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in light of the root cause discovered by rojer
and addressed in cesanta/dev#5882 for the http connection code path.

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CC3200 (TI's C library) doesn't like it

https://e2e.ti.com/support/development_tools/compiler/f/343/t/554883

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Digital signature key usage is required for the key to be used for (EC)DHE.

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Currently only OpenSSL API impl, but the plan is to add more.

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Causes issues on Windows - looks like "parallel" makefiles
do not handle spaces in directory names properly.

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And pre-create directories for SDK for both nRF51 and nRF52.

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Now all warnings are disabled on SDK files, and enabled on our
application's files.

Fixed a couple of things in nRF52 which were unnoticed because all
warnings were disabled there from the beginning.

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Unfortunately it turns out Nordic's examples were able to build with GCC
just because they suppressed all warnings; I had to do the same here
because there are some macro redefinitions of arm-none-eabi-gcc and
lwip.

TODO: resolve that and do not suppress warnings.

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Now it works in hardware.

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Long-lived UDP "connections" i.e. interactions that involve more
than one request and response are rare, most are transactional:
response is sent and the "connection" is closed. Or - should be.
But users (including ourselves) tend to forget about that part,
because UDP is connectionless and one does not think about
processing a UDP request as handling a connection that needs to be
closed. Thus, we begin with SEND_AND_CLOSE flag set, which should
be a reasonable default for most use cases, but it is possible to
turn it off the connection should be kept alive after processing.

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