bootstd: cros: Allow detection of any kernel partition

The existing ChromiumOS bootmeth only supports reading a single kernel
partition, either 2 or 4. In fact there are normally two options
available.

Use the GUID to detect kernel partitions, with the BOOTMETHF_ANY_PART
flag, so that bootstd does not require a valid filesystem before calling
the bootmeth.

Tidy up and improve the logging while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
[trini: Add missing select of PARTITION_TYPE_GUID]
Signed-off-by: Tom Rini <trini@konsulko.com>