bootstd: cros: Add a function to read a kernel

bootstd: cros: Add a function to read a kernel

The code to read the ChromiumOS information from the partition is
currently all in one function.

Create a new function which reads the kernel, assuming that the metadata
has been parsed.

For now this function is not used. Future work will plumb it in.

Signed-off-by: Simon Glass <sjg@chromium.org>