Linux build should be able to use Zpehyr SDK

Not listed in  the instructions, but was able to use the Zephyr build tools already installed on an Ubuntu system. Need to change the line 

```
# Set environment variables
set(ZEPHYR_TOOLCHAIN_VARIANT gnuarmemb)
```

to 

```
# Set environment variables
set(ZEPHYR_TOOLCHAIN_VARIANT zephyr)
```