x86: zboot: Move kernel-version code into a function

x86: zboot: Move kernel-version code into a function

To help reduce the size and complexity of load_zimage(), move the code
that reads the kernel version into a separate function. Update
get_boot_protocol() to allow printing the 'Magic signature' message only
once, under control of its callers.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>