cpu/esp_common: fix the dependency of the flash image on the ELF file

[gschorcht]

Contribution description

This PR fixes the dependency of the flash image on the ELF file for the ESP* make system.

Flashing an ESP board first requires the creation of a flash image from the ELF file. This is realized in the preflash target. However, the preflash target only depends on the variable BUILD_BEFORE_FLASH but on the ELF file. Therefore, the variable BUILD_BEFORE_FLASH must be set to the ELF file to ensure that when using multiple make processes, the compilation of the ELF file is completed before the flash image is created.

Testing procedure

• In examples/hello-world run make BOARD=esp32-wroom-32 -j flash term.
• Change the text in main.c, better yet print it in a while(1) loop.
• Run make BOARD=esp32-wroom-32 -j flash term again

Issues/PRs references

Fixes #13492

[benpicco]

Thank you for investigating this!
Now make -j flash works reliable as it should.

[kaspar030]

This broke "make flash-only".

[kaspar030]

This is on master:

[kaspar@ng riot/examples/hello-world (master)]$ BOARD=esp32-wroom-32 make flash-only
ESP32_SDK_DIR should be defined as /path/to/esp-idf directory
ESP32_SDK_DIR is set by default to /opt/esp/esp-idf
ccache: error: Could not find compiler "xtensa-esp32-elf-gcc" in PATH
make[1]: *** [/home/kaspar/src/riot/Makefile.base:108: /home/kaspar/src/riot/examples/hello-world/bin/esp32-wroom-32/application_hello-world/main.o] Error 1
make: *** [/home/kaspar/src/riot/examples/hello-world/../../Makefile.include:568: /home/kaspar/src/riot/examples/hello-world/bin/esp32-wroom-32/application_hello-world.a] Error 2

make flash-only is required by CI testing to not recompile...

[kaspar030]

Maybe FLASH_FILE should be the .bin? The compile worker could then create that.
We'd have to override the rule that creates the .bin to do the current preflasher stuff.

RIOT/Makefile.include

Lines 583 to 585 in 2444f06

	%.bin: %.elf
	$(Q)$(OBJCOPY) $(OFLAGS) -Obinary $< $@

[gschorcht]

I'm under pressure due to the online summer term so that I'm not able to spend time at the moment. Maybe, we should revert this commit for the moment and reopen issue #13492.