Add generic build manifest #90

Job	Run time
prepare	5s
build (x86_64, httpreply, qemu) / buildunikernel	26s
build (aarch64, httpreply, fc) / buildunikernel	34s
build (aarch64, httpreply, qemu) / buildunikernel	37s
build (x86_64, httpreply, fc) / buildunikernel	34s
buildgeneric (x86_64, httpreply-go) / buildapp	31s
buildgeneric (aarch64, httpreply-go) / buildapp	57s
build (x86_64, httpreply, fc) / buildimage	25s
build (aarch64, httpreply, qemu) / buildimage	18s
build (x86_64, httpreply, qemu) / buildimage	17s
build (aarch64, httpreply, fc) / buildimage	19s
build (aarch64, httpreply, qemu) / sign	18s
buildgeneric (aarch64, httpreply-go) / sign	25s
buildgeneric (x86_64, httpreply-go) / sign	17s
build (x86_64, httpreply, fc) / sign	19s
build (aarch64, httpreply, fc) / sign	23s
build (x86_64, httpreply, qemu) / sign	14s
manifestgeneric (httpreply, go) / manifest	18s
manifest (httpreply, fc) / manifest	22s
manifest (httpreply, qemu) / manifest	18s
	7m 57s

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