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.
The boot loader now knows when to go into bootstrap mode by
looking for a specific EEPROM value. Also updated code style
to match Contiki code style guidelines.
