Added the ability to save RAMPZ and EIND registers (Atmega2560 has it)
(cherry picked from commit 001281e45fcb0a2ef89d8f149776f41ae867731a)
Merge branch 'DuinOS_v0.6.x' of https://github.com/carlosdelfino/DuinOS into DuinOS_v0.6.x
updated to freeRTOS v7.4.0
repaired some errors saving the stack
Merge pull request #7 from niesteszeck/DuinOS_v0.6.x

updated to freeRTOS 7.4.0 and repair an error related to r0