Set Bluefruit LE advertising interval

[cstawarz]

By default, the BLE firmware on the Adafruit Feather M0 Bluefruit LE will switch from fast advertising (20ms interval) to low-power advertising (417.5ms interval) after 30 seconds. I found that, after the switch to low-power advertising, connection attempts from macOS devices often failed (despite the fact that such a switch is explicitly condoned in Apple's documentation).

These changes alter the default configuration of Bluefruit LE devices to always use the initial, 20ms advertising interval. They also introduce a new preprocessor macro, FIRMATA_BLE_ADVERTISING_INTERVAL, which can be set by other device configurations. Users who want to stick with the Feather M0's default advertising behavior can simply comment out the macro definition in bleConfig.h.

[soundanalogous]

Thanks!