Adds script to clean up all example directories
Adds script to clean up all example directories

Usage:
./scripts/clean-examples

Example:
```
➜   ./scripts/clean-examples
cleaning examples/hifive-riscv-32
cleaning examples/qemu-armv7M-32
cleaning examples/qemu-armv8A-64
cleaning examples/qemu-riscv-64
done!
```

Signed-off-by: Piyush Itankar <4a4ff89c9da587217bb1deb9fbe4ebeb84f02484@google.com>
Change-Id: Idac7a1d9dccc8e371c4ddf2a0db58fc135ab1ba0

google/esh,google/esh