When build ubuntu iso image, it will install grub-efi-amd64-signed
and grub-efi-amd64 packages. Both of the postinst script will try
to find root device and install grub which will definitely fail in
such a chroot environment.
So the workaround is to skip error and remove postinst script.
And confirm the package be installed successfully at last.
Change-Id: Ie0aecb212b22362046db55b5ad8c64c3211c28e5
Closes-Bug: #1491280
Co-Authored-By: Jane.zhang <jian.zhang8@hpe.com>
