Update for Go1.16

[andig]

After upgrading to Go 1.16 I'm seeing this due to the switch to go modules by default:

gokr-packer -overwrite=evcc_0.46.image -target_storage_bytes=1258299392 -hostname evcc -http_port 8080 github.com/gokrazy/serial-busybox github.com/gokrazy/breakglass github.com/andig/evcc
2021/02/21 15:45:19 packer.go:539: building [github.com/gokrazy/serial-busybox github.com/gokrazy/breakglass github.com/andig/evcc]
2021/02/21 15:45:20 gotool.go:77: getting incomplete packages [github.com/gokrazy/gokrazy/cmd/dhcp github.com/gokrazy/gokrazy/cmd/ntp github.com/gokrazy/gokrazy/cmd/randomd github.com/gokrazy/serial-busybox github.com/gokrazy/breakglass github.com/gokrazy/gokrazy github.com/gokrazy/kernel github.com/gokrazy/firmware]
go: downloading github.com/gokrazy/gokrazy v0.0.0-20210130175324-e6c441f6312f
go: downloading github.com/gokrazy/kernel v0.0.0-20210216084928-394669de618f
go: downloading github.com/gokrazy/firmware v0.0.0-20210217033035-1f48ff6fd000
go get: added github.com/gokrazy/breakglass v0.0.0-20210118084620-1a4768ba6975
go get: added github.com/gokrazy/firmware v0.0.0-20210217033035-1f48ff6fd000
go get: added github.com/gokrazy/gokrazy v0.0.0-20210130175324-e6c441f6312f
go get: added github.com/gokrazy/kernel v0.0.0-20210216084928-394669de618f
go get: added github.com/gokrazy/serial-busybox v0.0.0-20200521210859-b4f0c5c3f3aa
2021/02/21 15:45:42 write.go:149: writing boot file system
no required module provides package github.com/gokrazy/rpi-eeprom; to add it:
        go get github.com/gokrazy/rpi-eeprom
2021/02/21 15:45:42 packer.go:1073: [go list -f {{ .Dir }} github.com/gokrazy/rpi-eeprom]: exit status 1
make: *** [image] Error 1

I do not have gokrazy added as a dependency to my project as it's only pulled in for certain build tags.

I can workaround this by using

GO111MODULE=auto GOOS=linux GOARCH=amd64 go get github.com/gokrazy/rpi-eeprom

but I was wondering why it's specifically complaining on the rpi-eeprom and if that dependency couldn't be added as part of tools/internal/updater?