oe-pkgdata-util: Setup logging before calling prepare

oe-pkgdata-util: Setup logging before calling prepare

This avoids test failures like:

INFO - ======================================================================
INFO - FAIL [1.046s]: test_find_path (pkgdata.OePkgdataUtilTests)
INFO - ----------------------------------------------------------------------
INFO - Traceback (most recent call last):
  File "/media/build1/poky/meta/lib/oeqa/core/decorator/__init__.py", line 32, in wrapped_f
    return func(*args, **kwargs)
  File "/media/build1/poky/meta/lib/oeqa/selftest/cases/pkgdata.py", line 50, in test_find_path
    self.assertEqual(result.output, 'zlib: /lib/libz.so.1')
AssertionError: 'NOTE: Starting bitbake server...\nzlib: /lib/libz.so.1' != 'zlib: /lib/libz.so.1'
- NOTE: Starting bitbake server...
  zlib: /lib/libz.so.1
INFO - ----------------------------------------------------------------------

since there is corruption in the output. Setting the logging up before
calling tinfoil.prepare() resolves this.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>