build: update prebuilt chromium script

Prebuilt chromium: update for lollipop

Change-Id: I4e34feb26c1b4bec10fb751a10dbc52579d7cd07

Prebuilt chromium: Run a check for target device directory

this should fix the issue that arises if device fails and device is built again,
chromium will error out because it thinks the prebuilt files exist (which they are
not, directories are just created).

Signed-off-by: Ayysir <ayysir@aospal.com>
Change-Id: I595cab268e66c738ccae9900150b36d7f9626a6e