_idf_target had no S3 case, so every S3 FQBN (esp32s3 / XIAO_ESP32S3 /
nano_nora) compiled as plain esp32 (LX6) and could not boot the S3 QEMU
machine. Add _is_esp32s3 (covers all three FQBNs; nano_nora has no 's3'
token) + return 'esp32s3'; add the xtensa-esp32s3-elf toolchain to the
build PATH; place the S3 second-stage bootloader at flash offset 0x0
(like C3) instead of 0x1000 via 'is_c3 or idf_target==esp32s3' at the merge.
Requires the xtensa-esp32s3-elf toolchain in the image
(install.sh esp32,esp32c3,esp32s3) and a libqemu-xtensa with an
esp32s3(-picsimlab) machine.
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