cpu/esp8266: fix problems with ESP WiFi and migration to ztimer

[gschorcht]

Contribution description

This PR fixes the problem with all types of ESP WiFi netdev's (esp_now | esp_wifi | esp_wifi_ap = esp_wifi_any) that arose with the migration to ztimer.

With PR #17386, ESP implementations were migrated to ztimer. If periph_rtt is provided as feature, ztimer uses by default periph_rtt whenever possible.

Unfortunately, the periph_rtt implementation for ESP8266 is quite tricky and uses a combination of the completely undocumented ESP8266 RTC module and the FRC2 counter to support deep sleep. However, this FRC2 counter also seems to be used by the binary SDK libraries for the WiFi interface once it is enabled by any ESP WiFi netdev (esp_wifi_any). I already pointed out this problem when I provided the periph_rtt implementation (#13640 (comment) and #13640 (comment)). Due to the fact that the SDK libraries for the WiFi interface are closed-source and completely undocumented, nobody knows what actually happens in these libraries.

As a result of this conflict, ztimer must not use periph_rtt when esp_wifi_any is used. Therefore, ztimer_no_periph_rtt is enabled if esp_wifi_any is used. Since xtimer always used periph_timer instead, we never ran into a problem with xtimer and esp_wifi_any.

This PR also includes a fix for the linker script. ztimer_core functions have to reside in IRAM for timing reasons and to be available also when the IROM cache is disabled. Although the module is called ztimer, its object files are generated in directory ztimer_core.

Kconfig already takes the migration of ESP WiFi netdev into account.

Testing procedure

With commit fe72971 two test applications have been provided for compilation tests. They are copies of tests/shell but enable additionally ztimer and ztimer + esp_wifi, repsectively. app.config.test files also enable these modules. It will be removed before merge.

1. Compilation should succeed in CI.

2. Use for example commands:

   BOARD=esp8266-esp-12x make -C tests/shell_ztimer info-modules
   BOARD=esp8266-esp-12x make -C tests/shell_ztimer_esp_wifi info-modules
   

   and compare enabled modules:

   • ztimer_periph_rtt and periph_rtt are used for tests/shell_ztimer w/o esp_wifi
   • ztimer_no periph_rtt and periph_timer are used for tests/shell_ztimer_esp_wifi with esp_wifi

Issues/PRs references

Fixes #17386

[kaspar030]

ACK. Trusting your testing.

[gschorcht]

Thanks for reviewing and merging.