Fix partition file search

Fix partition file search

* Change file_block_open() to check for FILE_OK rather then NULL
return from fs_open().  When no partition is specified in a file open
call yaboot will loop through all known partitions until the file is
found, or no more partitions are left.  In older versions fs_open()
would check for FILE_OK, so checking for NULL return from fs_open()
worked, in current versions fs_open() checks for BADFS so proper
errors can be reported.  This only really affects IBM CHRP.

git-archimport-id: erbenson@alaska.net--public/yaboot--devel--1.3--patch-28