ENYO-3012: (less) fix case where `package.js` deep (>1) in the source tree

less-generated assets need to  be referred to from `build/index.html`,
not from the folder where the top-level `package.js` is living.

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>