SAMv7 Progmem driver byte write lead to ECC errors and flash data corruption

SAMv7 progmem (internal flash) driver does not perform byte writes correctly.
The driver assumes that it is possible to read page content, change erased (`0xFF`) bytes value and write page back to flash. That is usually a case for most of standard flashes, but SAMv7 internal flash has built-in ECC support, so applying such procedure destroys the flash content and rise "Multiple ECC Error" bit in `EEFC_FSR` register. The partial page programming of internal flash is of course possible, but with some restrictions specified in datasheet.

So only the first partial programming after erase is forking fine.
I found this and debugged for a while trying to place MTD config data on top of internal flash page.
Just logging it here as I do not plan to do anything with it, but this information may save investigation time for someone else.