Updates to Chapter 27S Installing Segger embOS

Please follow the directions in Chapter 27S of the 2nd Ed. of Learning the Art of Electronics. However, it appears the Segger IDE has trouble finding .h header files if installed on a remote disk such as google drive or dropbox. Install it on your local drive. In addition, Windows allows a maximum of 256 characters in a path/filename. The embOS directory structure is relatively deep, so if you have problems on Windows, try unzipping the RTOS download closer to your root directory.