Added support for picoTCP TCP/IP stack

This is an alternative to the default Linux TCP/IP stack, which is
saving >300KB in code size, while still providing TCP/IP connectivity.

How to set it up:
- Disable the TCP/IP from linux (CONFIG_INET=n)
- Activate picoTCP (CONFIG_PICOTCP=y) ... see menuconfig

What is currently working:
- device drivers
- setting up IP addresses via ifconfig
- setting up routes
- socket communication
- most common socket options
- socket ioctl()s
- getaddrinfo()
- setting duty cycle of the stack kthread via
  /proc/sys/net/core/picotcp_duty_cycle

What is not yet working:
- Raw sockets
- Netlink sockets (to use iproute2 ...) - WIP
- iptables integration (picoTCP support IP filtering)
- full IPv6 - WIP