AVR109: support for prodsig read and usersig read/write

[MCUdude]

This PR allows the AVR109 based Xboot bootloader to read prodsig and read/write to the usersig memory on any Xmega that Xboot supports. Tested on an ATxmega256A3BU.

This is an extension of the original AVR109 protocol, and I've not added any guards to prevent users from reading/writing to thee memories. Mostly because (I think) it's unlikely that Xmega users are running a different avr109 bootloader implementation.

Closes #200

[stefanrueger]

Looks cool, if it works (presumably, you tried?) should be merged.

[MCUdude]

Looks cool, if it works (presumably, you tried?) should be merged.

Thanks. I've tested it and can confirm that it works perfectly. It can perhaps be merged together with #1101 and #1102

[MCUdude]

@mcuee it would be great if you could try this PR on your Xmega hardware as well before merging, just to be sure

[mcuee]

@mcuee it would be great if you could try this PR on your Xmega hardware as well before merging, just to be sure

Sorry this is a bit tricky for me to carry out quick testing as my xmega breakout board is really just a breakout board. Please proceed with the merge. I will try to test this later.

[MCUdude]

Sorry this is a bit tricky for me to carry out quick testing as my xmega breakout board is really just a breakout board. Please proceed with the merge. I will try to test this later.

I think it's ready to be merged. I'll let Stefan do it together with the rest of the PRs that are also ready.
On the board I used for testing, an Xplained ATxmega256A3BU, I had to wire a USB to serial adapter directly to the UART0 pins and compile the code to match the board layout (start bootloader with SW0 switch, etc.)

[mcuee]

Please go ahead with the merge and not wait for my testting. Thanks.