xtimer: rework dependecies to ease backend switch

[fjmolinas]

Contribution description

This PR sparks from #17721 (comment), trying to make the default backend change as close to a oneliner as possible. Because of #17721 (comment) I actually had to rework dependencies quite a bit, so Murdock might not be happy again... let's see.

With this change the default could be change with the following.

diff

diff --git a/makefiles/boards/ztimer_only.dep.mk b/makefiles/boards/ztimer_only.dep.mk
index f50810a051..3b6b20b03a 100644
--- a/makefiles/boards/ztimer_only.dep.mk
+++ b/makefiles/boards/ztimer_only.dep.mk
@@ -5,7 +5,7 @@
 # ztimer_xtimer_compat is used.
 
 ifneq (,$(filter xtimer,$(USEMODULE)))
-  ifeq (,$(filter ztimer_xtimer_compat ztimer64_xtimer_compat,$(USEMODULE)))
-    USEMODULE += xtimer_on_ztimer
+  ifeq (,$(filter xtimer_on_ztimer,$(USEMODULE)))
+    USEMODULE += ztimer_xtimer_compat
   endif
 endif
diff --git a/sys/xtimer/Kconfig b/sys/xtimer/Kconfig
index 3fe4a309bb..d4d538b034 100644
--- a/sys/xtimer/Kconfig
+++ b/sys/xtimer/Kconfig
@@ -19,19 +19,20 @@ if MODULE_XTIMER
 config XTIMER_ZTIMER_BACKEND
     bool "xtimer always implemented by ztimer"
     select MODULE_ZTIMER_USEC
+    default y
 
 choice XTIMER_BACKEND
     bool "xtimer backend"
 
-config MODULE_XTIMER_ON_ZTIMER
-    bool "xtimer api implemented by xtimer, with ztimer_usec backend"
-    depends on MODULE_ZTIMER_PERIPH_TIMER
-
 config MODULE_ZTIMER_XTIMER_COMPAT
     bool "xtimer api implemented by ztimer"
     depends on MODULE_ZTIMER_USEC
     select MODULE_DIV
 
+config MODULE_XTIMER_ON_ZTIMER
+    bool "xtimer api implemented by xtimer, with ztimer_usec backend"
+    depends on MODULE_ZTIMER_PERIPH_TIMER
+
 config MODULE_XTIMER_ON_PERIPH_TIMER
     bool "xtimer api implemented by xtimer, with periph_timer backend"
     select MODULE_DIV
diff --git a/sys/xtimer/Makefile.dep b/sys/xtimer/Makefile.dep
index cc643e88ad..4f964ee4e1 100644
--- a/sys/xtimer/Makefile.dep
+++ b/sys/xtimer/Makefile.dep
@@ -9,7 +9,6 @@ DEFAULT_MODULE += auto_init_xtimer
 # order issues, this way xtimer_on_periph_timer can be turned into a DEFAULT_MODULE
 # that can be disabled if ztimer_periph_timer
 FEATURES_REQUIRED += periph_timer
-DEFAULT_MODULE += xtimer_on_periph_timer
 
 # To avoid inclusion order changing dependencies this dependency is commented out
 # but it models xtimer_on_periph_timer dependency explicitly, see comment above
@@ -18,12 +17,8 @@ DEFAULT_MODULE += xtimer_on_periph_timer
   # USEMODULE += div
   # FEATURES_REQUIRED += periph_timer
 # endif
-
-ifneq (,$(filter ztimer_periph_timer,$(USEMODULE)))
-  DISABLE_MODULE += xtimer_on_periph_timer
-  ifeq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
-    USEMODULE += xtimer_on_ztimer
-  endif
+ifeq (,$(filter xtimer_on_%,$(USEMODULE)))
+  USEMODULE += ztimer_xtimer_compat
 endif
 
 ifneq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))

Although adapting tests Makefile.ci will also be needed (17b3c6e and 6728d7c)

This PR also changes the auto-selection of the compat modules from ztimer to ztimer_periph_timer, it, therefore, allows having legacy timer and ztimer_msec running on rtt.

From local testing (To be confirmed by murdock), I think this change allows getting rid of the ZTIMER_USEC indirection as well as ZTIMER64_USEC indirection

Marking as WIP will I let Murdock check I didn't make a fundamentally wrong change.

Testing procedure

• Green Murdock

Issues/PRs references

#17721

[fjmolinas]

Closing in favor off #17811