Symlink all flash files into temp dir

- Depending on where the image to flash is sourced from, we may not be
able to create the symlinks for things like bl2.
- To solve this, symlink all files from the files_dir into a temporary
directory, and perform all other operations in that directory.
- Clean up the temp dir with an EXIT trap.

Change-Id: I44dd870719d84219c69d9983ff9281fd6b2a4f7a
1 file changed