bootstd: cros: Add a function to read info from partition

bootstd: cros: Add a function to read info from partition

The code to read the ChromiumOS information from the partition is
currently all in one function. It reads the entire kernel, which is
unnecessary unless it is to be booted.

Create a new function which reads just the minimum required data from the
disk, then obtains what it needs from there.

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

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