This adds compile-time checks for format strings.

Fix issues exposed by this.

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Configured via `debug.std{out,err}_topic`

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It's confusing

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And also fix `flash` target for nrf51

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This reverts commit 1a17e17c462bdd4e1d26d8742f8b7087273e04c2.

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URLs like mongoose-iot.com are left intact

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Fix errors so it builds.

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`MG_ENABLE_...` macros are not yet renamed because it involves some
logic change: e.g.  instead of defining `MG_ENABLE_MQTT`, one should
define `MIOT_ENABLE_MQTT`, which should imply `MG_ENABLE_MQTT` as well.

Macro refactoring is going to be done in another PR.

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

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