[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.
Merge pull request #782 from ejoerns/pull-req/avr-mcucsrfix

[avr] Removed usage of deprecated MCUSR