Improve serial command handling #1416

Job	Run time
build-frontend	1m 30s
getvars / get-vars	9s
build-firmware (Wemos-D1-Mini-ESP32)	54s
build-firmware (Wemos-Lolin-S2-Mini)	1m 12s
build-firmware (Wemos-Lolin-S3)	1m 15s
build-firmware (Wemos-Lolin-S3-Mini)	1m 13s
build-firmware (Waveshare_esp32_s3_zero)	1m 13s
build-firmware (Pishock-2023)	55s
build-firmware (Pishock-Lite-2021)	56s
build-firmware (Seeed-Xiao-ESP32C3)	1m 16s
build-firmware (Seeed-Xiao-ESP32S3)	1m 13s
build-firmware (DFRobot-Firebeetle2-ESP32E)	57s
build-firmware (OpenShock-Core-V1)	1m 10s
build-firmware (OpenShock-Core-V2)	1m 14s
build-firmware (NodeMCU-32S)	1m 9s
build-staticfs	35s
merge-partitions	0s
checkpoint-build	0s
cdn-upload-version-info	0s
cdn-upload-firmware	0s
cdn-bump-version	0s
checkpoint-cdn	0s
release	0s
	16m 51s

Provide feedback