[avr] Removed usage of deprecated MCUSR

Since avr-libc 1.8.0 MCUSR is marked as poison as it was replaced by the
correct name MCUCSR.
Thus code still using the old MCUSR name does not compile anymore.

This commit replaces usages of former MCUSR by its new name MCUCSR and
modifies the alias fallback accordingly.
Added missing memb_free in queuebuf.c
Fix deallocation in queuebuf.c, case where QUEUEBUF_STATS is set
[doc] fixed some minor typos
[doc] added base doxygen file for apps and cpu directory to doc/
[doc] moved doxygen parts from core/net/simple-udp.c to corresponding
header file
[doc] updated settings manager documentation
[doc] Various fixes for doxygen grouping etc.
[core/net/rime] Doxygen fixes
[core/net/mac] Doxygen: Added frame802154 to net group
[doc] added faulty esb and msp430 references
[core/net/rime] Avoid equal group titles in same group
[platform/native] Add to doxygen tree
[doc/cpu.txt] added pic32 and aducrf101 cpu
[doc/html/img/esb] deleted obsolete folder
[cpu] removed faulty group names
[doc/html] Removed .cvsignore
[cpu/arm] removed free-standing @addtogroup command
[doc] Fixed and simplified doc/cpu.txt
[cpu/arm] Added doxygen .txt files for gaining a minimum of documentation structure