Place the META/ dir first in the target_files.zip.

To help early verification of a target_files.zip being uploaded to our
servers, we place the META/ directory first in the .zip file, so checks
against the product_id.txt and product_version.txt don't need to wait
for the whole file to upload.

Note that META/*filesystem_config.txt files are generated and added to
the .zip file at a later point, so they are not included at the
beginning.

Bug: 26806325
TEST=`make dist` and `unzip -l out/dist/edison-target_files-eng.$USER.zip` shows most META/ files first.

Change-Id: I1955645412688f5c7823063ed56606b251daccfd