Assume only 32u4 boards can run 'promicro' converters (#23312)

builddefs/converters.mk

@@ -5,8 +5,10 @@ ifneq ($(findstring yes, $(CTPC)$(CONVERT_TO_PROTON_C)),)
 $(call CATASTROPHIC_ERROR,The `CONVERT_TO_PROTON_C` and `CTPC` options are now deprecated. `CONVERT_TO=proton_c` should be used instead.)
 endif
 
-# TODO: opt in rather than assume everything uses a pro micro
-PIN_COMPATIBLE ?= promicro
+ifneq (,$(filter $(MCU),atmega32u4))
+    # TODO: opt in rather than assume everything uses a pro micro
+    PIN_COMPATIBLE ?= promicro
+endif
 
 # Remove whitespace from any rule.mk provided vars
 #   - env cannot be overwritten but cannot have whitespace anyway
@@ -16,6 +18,10 @@ ifneq ($(CONVERT_TO),)
     # stash so we can overwrite env provided vars if needed
     ACTIVE_CONVERTER=$(CONVERT_TO)
 
+    ifeq ($(PIN_COMPATIBLE),)
+        $(call CATASTROPHIC_ERROR,Converting to '$(CONVERT_TO)' not possible!)
+    endif
+
     # glob to search each platfrorm and/or check for valid converter
     CONVERTER := $(wildcard $(PLATFORM_PATH)/*/converters/$(PIN_COMPATIBLE)_to_$(CONVERT_TO)/)
     ifeq ($(CONVERTER),)