board/system: add support for an optional /etc/rc.local #57

Job	Run time
supervisor (curios-nftables) / build (amd64)	3m 49s
supervisor (curios-ntpd) / build (amd64)	5m 18s
supervisor (curios) / build (amd64)	13m 36s
supervisor (curios-httpd) / build (amd64)	3m 49s
supervisor (curios) / build (arm64)	13m 54s
supervisor (curios-nftables) / build (arm64)	4m 14s
supervisor (curios-ntpd) / build (arm64)	5m 20s
supervisor (curios-httpd) / build (arm64)	3m 48s
supervisor (curios-httpd) / push	10s
supervisor (curios-nftables) / push	26s
supervisor (curios-ntpd) / push	17s
supervisor (curios) / push	31s
	55m 12s

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