Ensure sdkconfig.h is built early, by making Makefile depend on it

Here's why this helps fix the build error:
If this file does not exist or is out of date, then it is generated
and then the Makefile is re-read.

I don't know why this worked before but stopped now; it must have
to do with the rework of header generation in the MP build system.

Author: jepler

ports/espressif/Makefile

@@ -353,6 +353,7 @@ endif
 # create the config headers
 .PHONY: do-sdkconfig
 do-sdkconfig: $(BUILD)/esp-idf/config/sdkconfig.h
+Makefile: $(BUILD)/esp-idf/config/sdkconfig.h
 $(BUILD)/esp-idf/config/sdkconfig.h: boards/$(BOARD)/sdkconfig CMakeLists.txt | $(BUILD)/esp-idf
 	IDF_PATH=$(IDF_PATH) cmake -S . -B $(BUILD)/esp-idf -DSDKCONFIG=$(BUILD)/esp-idf/sdkconfig -DSDKCONFIG_DEFAULTS="$(SDKCONFIGS)" -DCMAKE_TOOLCHAIN_FILE=$(IDF_PATH)/tools/cmake/toolchain-$(IDF_TARGET).cmake -DIDF_TARGET=$(IDF_TARGET) -GNinja -DCIRCUITPY_ESPCAMERA=$(CIRCUITPY_ESPCAMERA)