sysupgrade: only update loader when necessary

To prevent unnecessary wear on NAND devices we want to only update bootloader when there is an actual update. For this purpose the bootloader is built with a magic string containing a version number identifying current bootloader version. The version of the new bootloader is checked from FIT. These can then be compared to determine if an update is needed.

Note: This solution only deals with the conditional update part. The version information is expected to be present (if not, version 0 is assumed). Adding the bootloader version information is a separate task.