fix libcoap checkout target

Fixes the following error:
make[1]: Entering directory '/foo/RIOT/pkg/libcoap'
git clone http://git.code.sf.net/p/libcoap/code /foo/RIOT/pkg/libcoap/libcoap && \
	cd /foo/RIOT/pkg/libcoap/libcoap && git checkout 89acaa6775ca3a4aedea510557e6b9c0c01fa5db
fatal: destination path '/foo/RIOT/pkg/libcoap/libcoap' already exists and is not an empty directory.
Makefile:25: recipe for target '/foo/RIOT/pkg/libcoap/libcoap' failed
replaced stack sizes in etx_beaconing by variables

resolves #356
Merge pull request #545 from OlegHahm/problematic_stack_sizes

replaced stack sizes in etx_beaconing by variables
configure debug target for native
shell: added a handler for mersenne twister
Merge pull request #558 from mehlis/shell-srand

shell: added a handler for PRNG
check for active_thread != NULL in syscalls.c before using it

fixes #498
Fixed some typos & language in pkg/PORTING
Merge pull request #589 from Lotterleben/porting-typos

Fixed some typos & language in pkg/PORTING
Fix my licensing
Merge pull request #587 from LudwigOrtmann/native_debug_target

configure debug target for native
Merge pull request #588 from LudwigOrtmann/issue_498

check for active_thread != NULL in syscalls.c before using it
Merge pull request #590 from authmillenon/licensing

Fix authmillenon's licensing
Merge pull request #578 from LudwigOrtmann/libcoap_makefile

fix libcoap checkout target
Add Oliver Gesch as author of his files and license header
Merge pull request #591 from authmillenon/author_oliver

Add Oliver Gesch as author of his files
set lpc2k_pgm return value

fixes the build not failing when flashing fails
remove cruft from obj dir, add gitignore
add obj/*.d to clean target
Merge pull request #592 from LudwigOrtmann/lpc2k_pgm

Lpc2k pgm