Support building EC firmware for Novacustom

To build EC, some changes are needed for Heads to build it and add it in rom from a Heads future commit so it's automatically flashed as part as internal upgrade alongside coreboot. Users should not care. The nix docker image will need to be updated with required deps and first build successfully. Following https://docs.dasharo.com/unified/novacustom/recovery/#ec-firmware-recovery Debian deps equiv to nix will be needed: - avr-libc avrdude binutils-avr clang-format clang-format-19 gcc-avr libcap-dev libftdi1 libhidapi-dev libhidapi-libusb0 libudev-dev libusb-0.1-4 sdcc sdcc-libraries (map to nix equivalents and put under flake.nix with high level deps which should install lower ones) - As well as cargo and probably others ----- Error on debian-13 following upstream docs : ``` user@debian-13:~/ec$ make BOARD=arduino/mega2560 CC arduino/mega2560/2026-01-12_6d9d6cf/arch/avr/gpio.o CC arduino/mega2560/2026-01-12_6d9d6cf/arch/avr/i2c.o CC arduino/mega2560/2026-01-12_6d9d6cf/arch/avr/i2c_slave.o CC arduino/mega2560/2026-01-12_6d9d6cf/arch/avr/uart.o CC arduino/mega2560/2026-01-12_6d9d6cf/board/arduino/mega2560/battery.o CC arduino/mega2560/2026-01-12_6d9d6cf/board/arduino/mega2560/i2c.o CC arduino/mega2560/2026-01-12_6d9d6cf/board/arduino/mega2560/main.o CC arduino/mega2560/2026-01-12_6d9d6cf/board/arduino/mega2560/parallel.o CC arduino/mega2560/2026-01-12_6d9d6cf/common/i2c.o CC arduino/mega2560/2026-01-12_6d9d6cf/common/keymap.o CC arduino/mega2560/2026-01-12_6d9d6cf/common/version.o LINK arduino/mega2560/2026-01-12_6d9d6cf/ec.elf OBJCOPY arduino/mega2560/2026-01-12_6d9d6cf/ec.ihx OBJCOPY arduino/mega2560/2026-01-12_6d9d6cf/ec.rom avr-objcopy: build/arduino/mega2560/2026-01-12_6d9d6cf/ec.ihx: invalid operation make: *** [src/arch/avr/toolchain.mk:24: build/arduino/mega2560/2026-01-12_6d9d6cf/ec.rom] Error 1 ``` Attached make -d output cc @mkopec (note that debian is not supposed to be supported, just info: [make_debug_on_debian_13.txt](https://github.com/user-attachments/files/25494851/make_debug_on_debian_13.txt) --- Input for https://github.com/linuxboot/heads/pull/2039 TODO: - [ ] add tooling in Heads so "System Information" can provide current EC version - [x] nix flake.nix maybe flake.lock will adapted with all EC builder needed dependencies - [ ] add target/mega2560.mk to build EC - [ ] boards configs adding target/mega2560.mk - [ ] coreboot configs point to ec.rom for coreboot configs to have ``` CONFIG_EC_SYSTEM76_EC_UPDATE=y CONFIG_EC_SYSTEM76_EC_UPDATE_FILE="ec.rom" ``` or DASHARO_EC counterpart? Something else @mkopec ?