SoftwareSerial should be optional for some boards

- Arduino board: Feather M0

- Arduino IDE version (found in Arduino -> About Arduino menu): 1.8.1

- List the steps to reproduce the problem below (if possible attach a sketch or
copy the sketch code in too):

The library does not build without `SoftwareSerial` support.

There should be preprocessor guards around all the `SoftwareSerial` functions similar to the `Adafruit_BluefruitLE_nRF51` Library:

```
#define SOFTWARE_SERIAL_AVAILABLE ( ! (defined (_VARIANT_ARDUINO_DUE_X_) || defined (ARDUINO_ARCH_SAMD) || defined (ARDUINO_STM32_FEATHER)) )
```

A reference commit from the `Adafruit_BluefruitLE_nRF51` repo: https://github.com/adafruit/Adafruit_BluefruitLE_nRF51/commit/bdad61148ee3a4a1a9c84bc76fcf962a60ca3b11

Start by locating all SoftwareSerial functions in the Adafruit-VC0706-Serial-Camera-Library and compare their handling with the referenced Adafruit_BluefruitLE_nRF51 commit. Done means the library builds for the Feather M0 without SoftwareSerial support while retaining support for boards that provide it.

Written by the indexing model from the issue text.