shell.nix: Add grub2 and dhcp

shell.nix: Add grub2 and dhcp

This is to allow manual testing of --dhcp-tftp --grub2 functionality.
Currently, I don't have time to fully automate the tests, but this is
how I tested it manually:

1. Create testboot script with the following content:

       #!/usr/bin/env novaboot

       BOOT_METHOD=linux
       load kernel
       load ramdisk

2. Create a bridge interface:

       brctl addbr br0
       ip link set dev br0 up

3. Run:

       ./testboot --dhcp-tftp --grub2 --remote-cmd="sleep inf" --netif=br0

4. Verify that the Qemu VM boots GRUB2:

       sudo qemu-system-x86_64 -boot n -nic bridge,br=br0 -nographic