Add gpio PIN constants

Also replaced defines with the constants where needed
in the main code
Main code clean up

Add using namespace hw to main.cpp
Add core periph constants to its own header file
Add PIN constants to gpio namespace
Add R constants to sysctl namespace
Remove constant defines from main.cpp
Remove unused commented code
Update code in main.cpp to work with moved constants
GPIO APB definition for port B
Simple code that enables UART1 on PB0, PB1