Fixes esp8266#427

Adds a basic I2S class based off of the Arduino-SAMD core.  The raw
i2s_xxx functions are still a better  way to use I2S due to their
flexibility, but this will allow basic Arduino sketches to work.

Use the enum type in the begin() method instead of generic int to
specify I2S formatting mode.

Remove tail recursion in the ::read() method.

Typo fix the NO_GLOBAL_ define to check for _I2S not _LITTLEFS.