add a one-argument version of package_extract_file

Add a version of package_extract_file that returns the file data as
its return value (to be consumed by some other edify function that
expects to receive a bunch of binary data as an argument).  Lets us
avoid having two copies of a big file in memory (extracting it into
/tmp, which is a ramdisk, and then having something load it into
memory) when doing things like radio updates.

Change-Id: Ie26ece5fbae457eb0ddcd8a13d74d78a769fbc70