improving hello world example & fix Readme dead links

[adrienluitot]

This is a little improvement of the Hello World example.
I replaced the call of the helloworld() by a printf(), I think it's more intuitive.

I also added a prototype of the bl_uart_init() function to make things clearer and explained a little more its parameters.

I finally took the liberty to change the baud rate, because with some test I figured out that if the baud rate is too high, some bugged characters are printed. I chose 230 400 because it's a multiple of the standard 9600 and seems to me to be a good compromise, it is fast enough and has no bugged character.

[gamelaster]

Hey, there is merge conflict.

Also, The baudrate works good on most of platforms.

[adrienluitot]

Hey, yep indeed @tchebb fixed the problem directly. ^^

For the baudrate I had less bugs with a lower one, but I can edit this if you want.

[adrienluitot]

I've just checked the conflict, I wanted to revert but I saw that the number of pages was not updated. But also when I made the PR the DS version 1.6 was not out, so I'm outdated on it. I don't know if you prefer that I update the readme for the number of page, or if you prefer that I revert and I let you do this ?

[uis246]

Most common baudrates are 9600 and 115200