Enable building ROM with custom keys

Updated build script to generate new keys if existing ones don't exist
along with requesting certificate information.

Feature: Generate and sign builds with new keys.
Updated build script to ask for certificate info and to generate new
keys if not available in build/target/product/security.

Use separate set of keys in device folder.
Updated to use keys in device/product folder instead of default
build/target/product.

Updated to follow workflow for automated builds and to allow for custom
signing option as well as default AOSP key signing option.

Updated gitignore to ignore device/mitre/svmp/security folder which
includes custom keys.

Updated svmp.mk to check if the environment variable of custom keys is
defined before setting the AOSP custom key variable.

Change-Id: Ibf522adabc35fffaae69ce7b8885f0659beb7907