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Factor out common part of SPIFFS libc interface support from the CC3200
port and reuse it for ESP32. Minor cleanups.

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This PR does not include `fw/platforms/pic32` since it's too dirty.

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Fix errors so it builds.

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PUBLISHED_FROM=00ca7b2adabde94a683ac4d33be1f2bdbb17772f

Namely, define it if only `MG_ENABLE_HTTP_STREAMING_MULTIPART` and
`MG_ENABLE_FILESYSTEM` are on

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PUBLISHED_FROM=2f6fb7ea5abdcd4ca9b06956172938ee3bc28459

And use it for `FW_ARCHITECTURE`. This is needed because with
Microchip's xc32, `-DFOO=\"bar\"` results in `FOO` being `bar`, not
`"bar"`, so we have to stringify differently.

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PUBLISHED_FROM=6abc158f198f5bd032e5ab06cc0ba3373434cdbe

On PIC32MZ, this is required to get `ntohl` and friends.

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PUBLISHED_FROM=26becdaf7dd61481b88b166ea87d44a7bc796660

Also performs trnasparent SPIFFS -> SLFS cert copy.

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

PUBLISHED_FROM=364caab21969ca28a05802c4dde93a11d1e0feda

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

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PUBLISHED_FROM=edb1063d3fd64e3c732f16b8bf101c065cdacddd

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

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PUBLISHED_FROM=46496c2d5340a0bbe4fe1c6c9ff187bc65d2d35f

PUBLISHED_FROM=774764f9d316e111912dae1fcaf2933141c59a60

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.

PUBLISHED_FROM=a33ab22d0ce85efea364b80478986b88f4b0a3a2

PUBLISHED_FROM=f8b4e8650d72c2ec5525e0510fbc8a80f8645a14

Now it works in hardware.

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