Below is the steps how to make grub udisk manually

my udisk is /dev/sdc

1. clean udisk information(mbr and partition information)

sudo dd if=/dev/zero of=/dev/sdc bs=1M count=1024 oflag=direct

2. create dos partition table

sudo fdisk /dev/sdc

o create a new empty DOS partition table
n -> p -> default->default->204800
a toggle a bootable flag
w write table to disk and exit

3. sudo mkfs.ext4 /dev/sdc1
   sudo mount /dev/sdc1 /media/wrsadmin/boot/
   sudo grub-install --root-directory=/media/wrsadmin/boot/ /dev/sdc

Installing for i386-pc platform.
Installation finished. No error reported.

4. vi grub.cfg, add your menuentry
   menuentry ‘wrlinux-nfs-dhcp-rootfs3’ {
   recordfail
   load_video
   gfxmode $linux_gfx_mode insmod gzio if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
   insmod part_msdos
   insmod ext2
   set root=‘hd0,msdos1’
   linux /boot/bzImage rootfstype=nfs root=/dev/nfs rw nfsroot=128.224.162.154:/nfsroot/rootfs3,vers=3 ip=dhcp console=ttyUSB0,115200n8
   }
   5.copy bzImage to /boot/bzImage
   6.set pc bios boot from udisk
5. about the rootfs
   for arm, you can build one with busybox by yourself;
   for x86, you can use yocto to build one.