grub: include additional grub utilities in final image

Reviewed By: no one

Add the grub-reboot and grub-set-default utilities to the ONIE
sysroot.

grub-reboot is used to reboot into a GRUB menu entry one time.  After
that one time the default menu entry is used.

grub-set-default is used to set the default GRUB menu entry
permanently.

Testing Done:

Exercised the utilities from the DEMO os using the kvm_x86_64 and
im_n29xx_t40n machine types.