Make sure that fdroid can find aapt in the current config, otherwise exit
with an error. Some users don't have build_tools set, and their SDK does
not include the build-tools in the default versioned dir, so this should
warn them of what is wrong.
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.
