soc: silabs: Support image properties for Series 2

Add image properties data structure to Series 2 binaries.
This data structure is used by the SE or bootloader to enforce
secure boot, and by other tools to extract image information.

Use the app version if set, or fall back to the kernel version
for the image version field. Set image type based on Kconfig
options.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>