grub: improve help text with detailed instructions to install grub

These instructions should probaby go in the Buildroot manual, but at
this point, it's not clear in which section they should go. So in
order to not loose those notes, add them to the Grub help text.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>