Samd on-board flash support.

[robert-hh]

I would like to discuss a software design decision. Some of the SAMD boards have in addition to the flash of the MCU an external flash device. For SAMx51 devices this is often connected to the QSPI interface. Meanwhile, I have a set of drivers for the internal flash, the QSPI flash and a SPI connected flash as C code. For the latter, I have as well a Python driver, which has about the same size and speed as the C version. The QSPI driver is based on an old Adafruit driver with a matching license. That all works well. Using the QSPI flash as default file system speeds up both the file system AND the code execution.
But, that's all a little different.

• The old MCU flash driver has a block device interface, while the QSPI and SPI flash drivers have a raw byte interface, and are used by a minimal Python block device driver to hook up to vfs. So I could change the interface for the MCU flash to a byte interface, and let it be accessed through the same block device driver. [Done]
• At the moment there are three classes in the samd module, samd.Flash, samd.QSPIflash and samd.SPIflash. I can change that to just Flash(id) with suitable IDs for the internal flash, the flash at the QSPI interface and the flash at a generic SPI interface.

So, what are the preferences? Any suggestions, @dpgeorge or @jimmo ?
B.t.w.: When I was almost finished with setting up the basic code, I came along the spiflash.c driver in micropython/drivers/memory, which seems to be used by the STM32 port. But I did not see how to use it, and the hardware interface was anyhow the major part of the driver.
P.S.: The actual state of the flash support is in the branch https://github.com/robert-hh/micropython/tree/samd_flash, pre-built images are here: https://github.com/robert-hh/Shared-Stuff/tree/master/samd_firmware