Add support for ATMega4809

fixes #55

Dynamically switch SPI port in use

This allows "derived" libraries (like the one in Arduino101 core) to use both the onboard or an external flash, while avoiding forks

Still not sure about SPIClass, is it universal? I coundn't find a core declaring it differently but I'm quite sure it exists. "typeof" SPI would be better.

RawHardwareTest.ino: Add chip ID for W25Q16DV

SerialFlash: enlarge chip ID buffer

SerialFlashChip::readID() writes up to 5 bytes into
the buffer provided, but the caller only allocates
3 bytes. Increase buffer size to 5 bytes.

Minor additions to example sketches for Arduino 101 compatibility

capacity: return 0 for wrong id

it checks for all bytes=0 or 0xff (the pin might be hold low or high by other connections)

Fix read-after-write

Fixes this problem:
https://forum.pjrc.com/threads/33594-SerialFlash-filesystem-question?p=101511&viewfull=1#post101511

(inportant, i think...)

Add readSerialNumber(uint8_t *buf);

Size of buf: 8 bytes

https://forum.pjrc.com/threads/33328-Prop-Shield-Beta-Test?p=98553&viewfull=1#post98553

Fix reading of filenames with length = multiple of 16

fix string_length

Update Readme for File Erase

...i don't know where this caused problems, but unpatched, it is wrong :)
I saw this when searching an other bug..

Bugfix blockSize (Spansion chip, model with 256K sector size)

The Spansion cards can have 64Kb or 256Kb depending on the model's
number. The device id[4] must be checked to determine the sector
architecture.