cpu/esp32: activate automatic XTAL detection

[gschorcht]

Contribution description

This PR changes the configuration of the ESP crystal frequency from a fix value of 40 MHz to crystal auto detection.

ESP32 can be clocked with either a 40 MHz or 26 MHz crystal. Since most boards use a 40 MHz crystal and the ESP32 design rules recommend a crystal frequency of 40 MHz, the configuration was previously set to 40 MHz. This commit changes the crystal configuration from 40 MHz to automatic detection, which allows also to use boards with a 26 MHz crystal like the Sparkfun ESP32 Thing DEV.

Testing procedure

If there is a board with a 26 MHz crystal at hand (I only know the Sparcfun ESP32 Thing DEV), flash and test any shell based application:

make BOARD=esp32-wroom-32 -C tests/shell

Without this PR, this should result into garbage at the console with the default UART console speed of 115200 baud but should work with an UART console speed of 74800 baud.

With this PR, the console should work with the default UART console speed of 115200.

If no board with 26 MHz is available, it should be sufficient to test that the board works as before.

Issues/PRs references

[benpicco]

Still works fine on esp32-wroom-32.
Are the bootloader changes always necessary when changing that define?
If so, that should be documented.

[gschorcht]

Still works fine on esp32-wroom-32.
Are the bootloader changes always necessary when changing that define?
If so, that should be documented.

No, I just had to change it the XTAL freqency detection in bootloaders to automatic to get it also working during the RIOT startup. Changing the define for RIOT back to a fix value has no affect on the bootloaders.

[gschorcht]

@benpicco Thanks for reviewing, testing and merging.