Changes:
* Create `nix/config.nix` with `config` defaults
* Add `nix/tools/gradlePropParser.nix` for reading `gradle.properties`
* Add `nix/mobile/android/keystore.nix` for generating a keystore
* Load keystore generation in `nix/mobile/android/default.nix`
* Use generated keystore if it's not provided via `config`
* Add `-deststoretype pkcs12` in `scripts/generate-keystore.sh`
* Add `nix/lib/assertEnvVarSet.nix` for checking if env var is set
Signed-off-by: Jakub Sokołowski <jakub@status.im>
Jakub Sokołowski