Bug#926149: AMD: Add nomodeset kernel parameter to avoid black screen

[積丹尼 Dan Jacobson <jidanni@jidanni.org>]

And lo and behold, in /etc/grub.d/10_linux :

if [ "$ubuntu_recovery" = 1 ]; then
    GRUB_CMDLINE_LINUX_RECOVERY="$GRUB_CMDLINE_LINUX_RECOVERY nomodeset"
fi