[BUG] Clock Delays is not working as expected

[pavly-gerges]

[Clock Issues]

_delay_ms(int) from util/delay.h isn't working as expected on arduino UNO (atmega328p) w/ 16MHZ external crystal on the HelloBlink example.

Steps to fix this:

1. Define a F_CPU to auto support external and internal resonators.
2. Add -O2 for optimizations level 2 for the avr-gcc.
3. Document the changes in the README.md file.

[pavly-gerges]

For Github, you can use this wiki on how to do a PR:
https://github.com/Google-Developers-Sohag/Github-Training/wiki/How-to-do-a-PR