converting tabs to spaces in core (#1439)

This PR converts tabs to white spaces.
The statement I used for the conversion:
'''find . -name "*.[ch]" -exec zsh -c 'expand -t 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;'''
Afterwards, I had a quick overview of the converted files to prevent odd indentation.

core/include/arch/hwtimer_arch.h

@@ -7,7 +7,7 @@
  */
 
 /**
- * @ingroup	    core_arch
+ * @ingroup     core_arch
  * @{
  *
  * @file        hwtimer_arch.h