It looks like Raspberry pi needs some time to get a digital pin
link properly after it is being exported.
Sample code to run:
```go
package main
import (
"fmt"
"github.com/kidoman/embd"
_ "github.com/kidoman/embd/host/rpi"
)
func main() {
defer embd.CloseGPIO()
fmt.Println(embd.SetDirection(10, embd.Out))
}
```
- before the change, the output is:
`open /sys/class/gpio/gpio10/direction: permission denied`
- after the change, the output is `<nil>`
Close#52
this ensures cleaner abstractions/code and will ensure that the produced
binary is as small as possible. a convenience package is provided to
easily load all hosts easily: "github.com/kidoman/embd/host/all"