task/kernel: fix grub2 detection

task/kernel: fix grub2 detection

grub2 rpm doesn't exist... el8 has

grub2-tools-extra-2.02-66.el8.x86_64
grub2-pc-modules-2.02-66.el8.noarch
grub2-tools-2.02-66.el8.x86_64
grub2-pc-2.02-66.el8.x86_64
grubby-8.40-34.el8.x86_64
grub2-common-2.02-66.el8.noarch
grub2-tools-minimal-2.02-66.el8.x86_64

and el7 has

grub2-tools-minimal-2.02-0.76.el7.centos.x86_64
grub2-pc-2.02-0.76.el7.centos.x86_64
grub2-common-2.02-0.76.el7.centos.noarch
grub2-tools-extra-2.02-0.76.el7.centos.x86_64
grubby-8.28-25.el7.x86_64
grub2-pc-modules-2.02-0.76.el7.centos.noarch
grub2-tools-2.02-0.76.el7.centos.x86_64
grub2-2.02-0.76.el7.centos.x86_64

The difference seems to be that el7 *also* has a /boot/grub directory. Let's
hope this change doesn't break it!

Signed-off-by: Sage Weil <sage@redhat.com>