base-files: sysupgrade: Do not set the iopsys_version for the other bank

Get rid of iopsys_set_iopversion_other(), as this is not needed anymore. This means that the iopsys_version property of the bank is not stored on its overlay anymore, and thus we do not need to set it at sysupgrade time anyway.

We still have to manage the iopsys_version cache though, which is used by get_iopsys_version(). For this, provide iopsys_clear_iopversion_cache() which clears the cache for the other bank only, to be called by platform_do_upgrade().