This allows the user to set the path to their Android SDK from the command
line. This option is named after the standard env var ANDROID_HOME, as used
in the build.xml generated by `android update project`. --android-home
takes precendence over the ANDROID_HOME env var if it is set.
This assumes that the smartcard is already setup with a signing key. init
does not generate a key on the smartcard, and skips genkey() if things are
configured to use a smartcard.
This also does not touch APK signing because that is a much more elaborate
question, since each app is signed by its own key.
