restructured bootloader logic

bootloader responds to serial commands on all resets
bootloader enters OTA routines only after watchdog reset
serial and OTA bootloader timeout logic improved
serial timeout ~ 200ms
OTA timeout ~ 10 seconds
both bootloaders ignore garbage inputs and jump to main app
added "OTA request" flag, reset reason feedback to main app via GPIOR0

preliminary boot image supplied for testing.
see #16 for discussion on remaining quirks
OTA request flag, reset reason added (now with added sauce)

source code for previous commit, plus additional fixes to logic flow
fixed bug preventing BL from jumping to main on first-ever serial download
Source code for release 0.50
release 0.51 - extra check to prevent from listening to serial traffic during OTA update
Merge branch 'master' of https://github.com/Pinoccio/core-pinoccio

Merge in bootloader-restructure changes

Conflicts:
	avr/bootloaders/STK500RFR2/STK500RFR2.atsuo