CL: Add mgos_gpio_blink()

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mbedTLS is just another lib now

CL: Use mbedTLS from mongoose-os-libs/mbedtls

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 * Update CubeL4 to 1.13.0
 * Rebuild OurTLS with `-DMBEDTLS_X509_CA_CHAIN_ON_DISK`
 * Add `-Wextra` and fix build issues

CL: Update STM32 build image

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CL: STM32: Make LWIP optional

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Can be used for cases where device has no networking but mongoose is still needed for its event loop.

CL: mg: Add net_if_null, a no-op net interface

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 * Change return type of mg_mgr_poll to return number of events
 * Add mg_mgr_min_timer
 * Refactor main poll loop to remove LwIP-specific stuff

CL: Refactor mg polling

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A smattering of other related changes

CL: STM32 OTA support for L475-IOT01 and DISCO-F746

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CL: Make esp8266 compilation a little less verbose

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Closes https://github.com/cesanta/mongoose/pull/945

CL: Fix build with CS_ENABLE_STDIO=0

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CL: mos support for LFS (littlefs) https://github.com/ARMmbed/littlefs

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Set CLOSE_IMMEDIATELY flag when destroying conn.
This avoid some races in LWIP adapter.

CL: mg_lwip: Fix race during clean conenction teardown

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startup cleanup on CC3220

CL: none

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CL: Fix TZ on ESP8266

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Enables extra compile-time checks

CL: none

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Also remove `-O3` optimization flasg, it negates `-Os`. Saves ~400 bytes.

CL: none

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CL: Apache license to common/

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A major cleanup, disentangling net_if and ssl_if.
Pulled a lot of common logic into the core and reduced size of net_if implementations.

CL: Mongoose net_if and ssl_if refactoring

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In preparation for making moving to lib.
Reduce amount of makefile copypasta a bit.

CL: none

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E.g. if `ASSERT_EQ(myfunc(), 123)` fails, `myfunc()` would be invoked again to print the actual value.
This can cause confusion sometimes.
To avoid this, we cast arguments to double, which is wide enough for most cases and we provide a variant for 64 bit value types for when it's not.
We also perform a check and fail if argument to the check results in loss of precision.

CL: none

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CL: none

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CL: none

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To properly fail a test in a subroutine, return value needs to be passed up.
This snippet makes it short.

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