Allow get-prebuilts to work from any CWD

The unzip command in get-prebuilts assumed that the CWD
as ./vendor/cm.  This changes allows it to work from any
CWD so now you can do

  ./vendor/cm/get-prebuilts

instead of

  cd ./vendor/cm && ./get-prebuilts

Change-Id: I705191fd84c3ffda6b2e81b7ea7518327cce8081
1 file changed