rom_explicit_buy() function

Buy an image Perform an "explicit" buy of an executable launched via an IMAGE_DEF which was "explicit buy" flagged. A "flash update" boot of such an image is a way to have the image execute once, but only become the "current" image if it calls back into the bootrom via this call. This call may perform the following: - Erase and rewrite the part of flash containing the "explicit buy" flag in order to clear said flag. - Erase the first sector of the other partition in an A/B partition scenario, if this new IMAGE_DEF is a version downgrade (so this image will boot again when not doing a "flash update" boot) - Update the rollback version in OTP if the chip is secure, and a rollback version is present in the image. NOTE: The device may reboot while updating the rollback version, if multiple rollback rows need to be written - this occurs when the version crosses a multiple of 24 (for example upgrading from version 23 to 25 requires a reboot, but 23 to 24 or 24 to 25 doesn't). The application should therefore be prepared to reboot when calling this function, if rollback versions are in use. Note that the first of the above requires 4 kiB of scratch space, so you should pass a word aligned buffer of at least 4 kiB to this method, or it will return BOOTROM_ERROR_INSUFFICIENT_RESOURCES if the "explicit buy" flag needs to be cleared.