add poweroff command

hc

2023-12-14 982b8cc116118b3463d3f332581945625722acd8

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# Locally computed
sha256    4a2af9d9c3ff488e92fb75b4ba38b35bcf9b8a66df04773eba2a7bbf1fa7529d    libserialport-0.1.1.tar.gz