CB-7818 - CLI builds ignore Distribution certificates (closes #114)

Fixed build / xcconfig defaults for CI environments. "cordova build -…
…-device --release" now builds using the "iPhone Distribution"-type profile, as opposed to the default "cordova build --device", using the default "iPhone Developer"-type profile.

Re-added build.xcconfig as shared .xcconfig for legacy compatibility. Implemented Configuration-level .xcconfig selection via inheritance. Added ASF comment section, short documentation.

Signed-off-by: Shazron Abdullah <shazron@apache.org>