With the new toolchain version required to fix issue #13133, the compilation of `examples/posix_socket` fails due to a C linkage error in `atomic_base.h`. The reason is that including `drivers/include/mtd.h` in `boards/esp32/board_common.h` inside the `extern C` block finally leads to including `atomic_base.h` inside the `extern C` block which in turn to the C linkage error for the template definitions in this file.
Some ESP32 boards (like my SparkFun ESP32 Thing) have a main clock
crystal that runs at 26MHz, not 40MHz. RIOT appears to assume 40MHz.
The mismatch causes the UART to not sync properly, resulting in
garbage written to the terminal instead of log output.
I’ve added:
* A new board configuration constant ESP32_XTAL_FREQ that defaults
to 40, but can be overridden by a board def or at build time to
force a specific value (i.e. 26).
* Some code spliced into system_clk_init() to check this constant and
call rtc_clk_init() to set the correct frequency.
* A copy of the rtf_clk_init() function from the ESP-IDF sources.
Fixes#10272
