add poweroff command

hc

2023-12-14 982b8cc116118b3463d3f332581945625722acd8

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# Locally computed:
sha256  938f597394b33e82e5af8c98bd5ea1a238f61892aabef36384adbf7ca5b52dda  u-boot-2018.01.tar.bz2