Implemented load of OSCCAL value (draft)

[LadislavOzobot]

Based on discussion with @ondrej-stanek-ozobot , this is attempt at implementing osccal in the bootloader. Would solve issue #87 The idea is originally by @WestfW in #342 (comment), this is the implementation i came up with:

• Any compiled bootloader starts with rjmp start_main and OSCCAL is not applied (MCU core loads factory default at startup and we don't touch it). This make sure the cpu can start up before the calibration is performed, and for example run a calibration program.
• rjmp start_main opcode is 0x05 0xc0, where 0x05 is the offset.
• when new osccal value is known, the code is patched:
  • ldi r24,0xFF instruction is patched to replace 0xFF with the new osccal value
  • rjmp start_main to rjmp set_osccal. This can be patched, as set osccal has relative address 0x05 only needs to modify 1s to 0s to become set_osccal 0x01.
  • from now on, the bootloader jumps to set_osccal before executing main and applies the new OSCCAL value
• ret and rjmp instructions are the same length, so it should work both with APP_NOSPM and without. But with NOSPM, it's probably necessary to flash it using a programmer, as there is no access to this region from program.

Caveats:

• I'm not sure if the code is stable enough across supported platforms. Specifically, I think there is different STS instruction on avrtiny core that is 1 word instead of 2. But they don't seem to be supported anyway?
• It wastes 1 word for the NOP, as I needed to be able to rewrite the address from 0x05 to 0x01. Making sts and ldi combo shorter would also help, the address just needs to be an even number. Or the rjmp instruction replaced for a different instruction? Not sure about that.
• I'm not AVR expert and I probably made a mistake.
• It does not solve how to put the calibration value into the right place. A separate program needs to handle that.

We will test on our platform and report if it works as expected. For now, we can have a discussion if this approach on the bootloader side is OK-ish and what more needs to be done.

Feel free to discuss, modify and reject if necessary :-)

Edit: formating.

[LadislavOzobot]

We have implemented the part that writes calibration using do_spm stub and it seems to work fine on atmega328 target. Is there interest in this PR? If so, let me know, I'll add documentation and example calibration code.