wolfBoot_update_trigger: Fix double-write-after-erase in NVM...WRITEONCE

[danielinux]

when wolfBoot_update_trigger is called on a device with NVM_ write-once workaround, it will cause an illegal second write to flash to set the UPDATING flag.

Instead, write the flag and the trailer in NVM_CACHE before committing to the flash.

Reported in zd19815