shell: refactor default shell command handling

before, we actually duplicated code for handling of shell commands
supplied to shell_init and those hardcoded by MODULE_SHELL_COMMANDS.

This patch refactors shell.c find_handler and print_help so there's
no duplicated code.
initial support for the mbed NXP LPC168

 * http://mbed.org/platforms/mbed-LPC1768/
convert into host byte order in UDP reception
make TCP in destiny optional
udp_packet_handler uses printf, stack must be bigger
fix copy&paste typo
moved stack from header to C file
 set message type for lowpan and ipv6 notifier
added IPv6 upper-layer checksum calculation
calculate RPL checksum right before sending the ICMP packet
make use of the IPv6 upper-layer checksum calculation
filled up IPv6 netmask correctly
hwtimer.h remove cruft, document, rearrange

remove unsused hwtimer_t0* declarations
document hwtimer_now
move hwtimer_now outside the internal section
initializes watchdog properly for MSB-A2

It the watchdog does not reset the WDMOD, BSL pin remains ignored.
Add package porting infomation
include version string
notify registered threads about _all_ received IP packets

(Before notifications were only sent for IP packets that were not
forwarded.)
pointer handling in lowpan notifier was wrong
another NTOHS was missing...
set received parent lifetime when creating a new parent