make calloc "usable" early on

closes #741

Old versions of the gnu libc uses calloc to allocate dymanic memory
when some error occurs in dlsym.
This results in a segfault as natives calloc wrapper has not been
initialized yet.
As this is a circular dependency and the libc can cope with this, we
just return NULL from the calloc wrapper and hope for the best.

Recent libc versions use a static buffer instead.
Fix at86rf231 driver
Fix cc2420 driver
Fix coding conventions in cc2420 driver
fixed includes for at86rf231
Merge pull request #762 from LudwigOrtmann/issue_741

make calloc "usable" early on
Merge branch 'fix-802-15-4-drivers' into tmp-merge-pr-644-740
Fix at86rf231 driver
Fix cc2420 driver
Fix coding conventions in cc2420 driver
fixed includes for at86rf231
Merge branch 'fix-802-15-4-drivers' into tmp-merge-pr-644-740

Conflicts:
	drivers/cc2420/cc2420_rx.c