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.
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
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.
include version string
another NTOHS was missing...
set received parent lifetime when creating a new parent
initializes RPL instances
printf is used in sixlowpan mac layer, stack needs to be adjusted
use local packet_length variable in sendto

Having a global packet_length variable doesn't seem to be the best
idea. To avoid trouble, a scope local variable should be preferred.
remove vtimer object before (re)scheduling it
check if preferred parent was set before comparing