This adds the hello-world process in hello-world.c to the platform build,
printing "Hello-world" to stdout on startup.
The entire platform is built, with uip stack, radio drivers, routing, etc.
So it is not usually a simple build!

When switching between ipv4 and ipv6 builds on a platform,
 $make TARGET=<platform> clean
else the library will contain duplicate or unresolved modules.

For example, using a loopback interface with the minimal-net platform:
[see http://www.sourceforge.net/apps/mediawiki/contiki/index.php?title=Capturing_loopback_traffic_with_Wireshark]
 $make TARGET=minimal-net
 $./hello-world.minimal-net
Hello, world
IP Address:  10.1.1.1
Subnet Mask: 255.0.0.0
Def. Router: 10.1.1.100
^C
 $make TARGET=minimal-net clean
 $make UIP_CONF_IPV6=1 TARGET=minimal-net
 $./hello-world.minimal-net
Hello, world
IPV6 Address: [aaaa::206:98ff:fe00:232]
IPV6 Address: [fe80::206:98ff:fe00:232]
^C

Note to AVR Raven users: Output goes to UART1, not the LCD. To see it,
 $make TARGET=avr-raven hello-world.elf
Load the .elf in AVR Studio and connect a hapsim terminal to the 1284p simulation.