1st, Why you don't use grub2-fll-fromiso?
2nd, without an entry root= in your kernel line, it can't work.
3rd, the manual speaks only about linux-fs for fromiso.
If you speak about a usbstick, which contains the iso, then some other questions:
Why you don't use dd for putting the iso to the stick? or
Why you don't use fll-iso2usb?