devices: z2_plus: change download folder to z2_plus

  * Currently there are 2 download folders: z2 and z2_plus. We were using z2 as
  it was the legacy folder before the codename was changed, but it doesn't
  really make any sense. So change it to z2_plus.

Change-Id: Ie563e3101ac681426f56a13a4916dd2c987af70b
1 file changed