feat(ulp): build ULP full subprojects as CMake v2 children

components/esp_hal_regi2c/CMakeLists.txt

@@ -11,6 +11,15 @@ if(EXISTS "${CMAKE_CURRENT_LIST_DIR}/${target}/include")
     list(APPEND includes "${target}/include")
 endif()
 
+# TODO(IDF-15950): potentially remove __ULP_BUILD handling from esp_hal_regi2c
+if(__ULP_BUILD)
+    # ULP ADC support reaches REGI2C headers through ADC HAL includes used by
+    # ulp/ulp_riscv/ulp_core/ulp_riscv_adc.c.
+    idf_component_register(INCLUDE_DIRS ${includes}
+                           REQUIRES soc hal esp_rom)
+    return()
+endif()
+
 # Per-target regi2c implementation sources (replaces ROM implementation when needed)
 if(NOT CONFIG_ESP_ROM_HAS_REGI2C_IMPL)
     list(APPEND srcs "${target}/regi2c_impl.c")