devtool: package: don't try to initialise tinfoil twice

devtool: package: don't try to initialise tinfoil twice

setup_tinfoil() already calls prepare(), we don't need to call it again
ourselves and doing so with tinfoil2 results in "ERROR: Only one copy of
bitbake should be run against a build directory". Calling prepare()
twice should probably still be allowed, so that ought to be fixed
separately, but in the mean time this code is still wrong so fix it
here.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>