ENYO-2060: Accept to pass non-Enyo Bootplate apps to PhoneGap build

- When no top-level `package.js` is found, minification is skipped &
  the entire source tree is passed to PhoneGap Build.

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>