Do not use ISRs and poll for interrupt flags in the main loop.
No need for moving vector table into bootloader and back.
Remove vector table by adding LDFLAGS = -nostartfiles.
Manually add some code which is now removed, but still needed by C
runtime:
- make sure that r1 is 0x00 (zero register).
- on some MCUs the stack pointer is not initialized correctly after
reset.
- main() need to be placed in special section .init9 to be called at all.
Not sure why the BSS and DATA segment are still initialized.
This change is heavily based on the optiboot bootloader.
TWEA bit in TWCR register needs to be cleared to send NACK.
Clear it when too many bytes were received, re-enable it after
when going idle.
Add missing state for prev. send NACK.
Catch all invalid states and reset bus.
TWINT bit needs to be set every time in ISR.
