Come to think of it, this exact issue has been reported several times in this thread. I believe they all had Chromeboxes as well. The error message you get is from the bootloader, it can't find a configuration file (i.e /flash/syslinux.cfg or /flash/EFI/BOOT/syslinux.cfg).
As for resizing your partition, there are a million ways... "parted" is included in this build. "Easy" is subjective.