Also performs trnasparent SPIFFS -> SLFS cert copy.

If the file's extension is .pem, convert it to DER format and put on SLFS.

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Also fixed a memory leak on reconnect: previously, each reconnect was
creating a `struct mg_connection` which was never reclaimed

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

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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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Now it works in hardware.

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Also, do not require -DRTOS_SDK and pick socket/low-level event manager
based on LWIP_SOCKET.

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This event is sent at the end of the request

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(It compiles, but doesn't really work yet)

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