Provide some manifest config option for GRUB_TIMEOUT

[olberger]

It seems that currently, the grub.cfg generation sets hardcoded values for GRUB_TIMEOUT and other grub options.

Debugging the boot process under virtualbox, for instance isn't easy when grub starts immediately without letting one optionally change the kernel boot arguments, by pressing a key before grub starts the kernel.

Thus, it should be handy to be able to define a different default than 0 for GRUB_TIMEOUT in the manifest.

Thanks in advance.