removing possible invalid check

Checking for a link local address to determine if a node is on-link is
probably not a valid assumption in most wireless networks.
native: don't use RIOT read/write in UART
Added standard way to query CCA status on CC2420 transceiver (squashed)
Completed CC2420 constants documentation (DOxygen)
Fixed error caused by non-inclusion of cc2420_txrx() prototype
Fixed implementation of channel_clear() function
Change year of file copyright
cpu: satisfy compiler in crash.c
Merge pull request #892 from OlegHahm/crash_noreturn

cpu: satisfy compiler in crash.c for ARM and MSP430
Fix for WSN430 1.4 driver
fixup for #710: use subfolder in bin dirs (native)

Update native Makefiles to provide changed object target paths
Fixes #989
Merge pull request #992 from LudwigOrtmann/fixup_710

fixup for #710: use subfolder in bin dirs (native)
Merge pull request #873 from OlegHahm/on_link_check

net: sixlowpan: removing possible invalid check
Fix length of fragmentation buffer in 6LoWPAN
Merge pull request #870 from authmillenon/fragbuf

sys:net:sixlowpan: Fix length of fragmentation buffer in 6LoWPAN
Merge pull request #915 from rousselk/radio-cca

Add a standard way to query CCA status on CC2420 transceiver.
Merge pull request #946 from Kijewski/native-write

native: don't use RIOT read/write in UART
Make: exterminate 'clean' buildtarget

Closes #993.

We do not need to descend into the modules to know what to do on
`make clean BOARD=blub`. We can just invoke `rm -rf bin/blub`.

This PR only keeps the descending into the USEPKGs, since they might
want to delete cached/downloaded/extracted data.