Fixed algorithmic error in ringbuffer.h
This caused the ringbuffer to be unable to not report correct
values when rb_isFull() is run.

Also fixed error in usart code where UDRE interrupt was
disabled/enabled where it should have been the interrupt flag
that should be raised/lowered.

The board is no longer crashing when run with buffered output
but it does not behave as expected, therefor issue #2 still applies.
When testing with the io_read_write test node that has a soft delay
at start of it's mainloop it appears as if this delay is prolonged
when using buffered output, as in it prints much slower than the
delay specifies.
Forgot renaming a macro on last commit
Merge branch 'master' into issue2
Merge branch 'master' into issue2
Fixed issue #2 for buffered input on usart1
Removed left over includes from debugging
Better code formatting in usart.c
Renamed uart usart where it was still missing
Added some more breathing space around if statements
in usart.c
Added doxygen grouping in usart.h
Merge branch 'master' into issue2
Verified that the buffered usar echo example works
So we can close issue #2