Add External Configuration

[gurudvlp]

Description

This pull request allows the use of an external configuration file for the build script's variables. After building several test images for the Banana Pi BPI-R3, it became apparent that I consistently installed additional packages, changed the timezone, and had to reset the default password. By sourcing an external config script, end users are able to have their own configuration while still falling back to default values.

This change includes:

• Checking for the existence of 'config.sh' and sourcing the file if it exists.
• Adding '/config.sh' to the .gitignore file.
• Adding 'config.example.sh' to demonstrate various ways that default values can be overridden.

Code Changes

The code changes in this pull request are exceedingly simple. There is an added check for the existence of 'config.sh', which if exists, is sourced. This is done after default values are set, so readability is not impaired, and anything not defined in 'config.sh' is still set with a sane default.

[ericwoud]

I had something very similar in the script before, but it was not really used. Can add it when I have some time available

I finally moved it a bit, so that config.sh can also contain ${target} and ${atfdevice}.

I did not include the example, as I like to keep things to a minimum.

Anyway, thanks for the input. I have also included in the rockchip version of the script.