Improve serial command handling #1504

Job	Run time
build-frontend	1m 30s
getvars / get-vars	16s
build-firmware (Wemos-D1-Mini-ESP32)	1m 32s
build-firmware (Wemos-Lolin-S2-Mini)	1m 50s
build-firmware (Wemos-Lolin-S3)	1m 41s
build-firmware (Wemos-Lolin-S3-Mini)	1m 52s
build-firmware (Waveshare_esp32_s3_zero)	1m 39s
build-firmware (Pishock-2023)	1m 25s
build-firmware (Pishock-Lite-2021)	1m 24s
build-firmware (Seeed-Xiao-ESP32C3)	1m 33s
build-firmware (Seeed-Xiao-ESP32S3)	1m 37s
build-firmware (DFRobot-Firebeetle2-ESP32E)	1m 34s
build-firmware (OpenShock-Core-V1)	1m 52s
build-firmware (OpenShock-Core-V2)	1m 50s
build-firmware (NodeMCU-32S)	1m 32s
build-staticfs	0s
merge-partitions	0s
checkpoint-build	0s
cdn-upload-version-info	0s
cdn-upload-firmware	0s
cdn-bump-version	0s
checkpoint-cdn	0s
release	0s
	23m 7s

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