grub2: Specify boot partition

Since boot partition was not specified, grub tools try to detect it
automatically. This patch add an option to force it.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>