To avoid conflicts on receiption of a frame while sending, _esp_wifi_recv_cb should not executed while a frame is in transmission

Overriding ethernet_input function _esp_wifi_recv_cb is only used when the WiFi interface is in connected state.

wifi_station_connect and wifi_stattion_disconnect never produce errors. Thus, error handling can be removed.

Different connection states defined to be able to realize a better connect/reconnect handling

Some SDK interrupt service routines obviously use malloc/free in the interrupt context. Because the rmutex-based lock/unlock approach of the malloc/free function suite of the newlib does not work in the interrupt context, the SDK memory management functions MUST therefore be used. To use the same memory management functions in RIOT as in the SDK, the malloc/free function suite has to be replaced by wrapper functions.

Since complete MAC frames are handled, ETHERNET_MAX_LEN has to be used instead of ETHERNET_DATA_LEN for receive buffer size and length check.

Fixes sporadic blocking of the wifi thread in esp_wifi_recv_cb function under heavy network load conditions when frames are coming in faster than they can be processed. Since esp_wifi_recv_cb function is not executed in interrupt context, the msg_send function used for ISR event can block when the message queue is full. With this change esp_wifi can be flooded with icmpv6 packets of maximum size without any problems over hours.

examples/bindist: cleanup and fix formatting in README

gnrc_mac: add timeout module.

dist/tools/compile_and_test_for_board: add compile and test script

cpu/saml1x: add support for SAML10 and SAML11 MCUs (Cortex-M23)

cpu/atmega_common: refactor AVR libc code into module

cpu/esp8266: change of ETS task handling 

tests/posix_semaphore: private sub functions for tests libs

tests/periph_dma: add automatic testing of DMA over UART

cpu/esp32: fix of buffer sizes and length checking in esp_eth and esp_wifi

Disable optimization for this function only with CPU_SAML1X MCUs due to an internal crosscompiler bug

Add SAML10/SAML11 support through SAM0 because hardware IP are the same so reuse to avoid duplication

boards/stm32: revert forced use of dma feature in STM32 boards

During flash write access, the IROM cache cannot be used and is disabled therefore. During that time, ets_post crashes if a functions is called which is not in IRAM. Therefore thread_flags_set must not be called if IROM cache is disabled.