arduino-cli FQBNs may carry a menu-option suffix
(vendor:arch:board_id:Menu1=val1,...). The espidf lane previously derived
cdc_on_boot exclusively from the board id's boards.txt default, so a
generic-devkit board could never be built with Serial on the USB
Serial/JTAG CDC port.
- _fqbn_board_id_and_options(): split board id and menu options, keeping
the old last-segment behaviour for plain 3-part FQBNs.
- _arduino_board_flags(): CDCOnBoot=cdc overlays cdc_on_boot=1 on the
boards.txt base flags (mirroring <id>.menu.CDCOnBoot.cdc.build.
cdc_on_boot=1); the per-board cache stores only base flags, overrides
are applied on a copy.
- _arduino_variant(): board id extraction goes through the same helper so
a menu suffix no longer corrupts the variant lookup.
First user: the ESP32-C3-LCDkit board def pins
esp32:esp32:esp32c3:CDCOnBoot=cdc — the real kit has no UART bridge, its
only console is the USB Serial/JTAG port, and the esp32c3js engine now
models the always-attached host that HWCDC's connection gate requires
(esp32c3js v1.8.0).
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