templates: Update grub script template files

templates: Update grub script template files

Update grub-mkconfig.in and 10_linux.in to support grub-probe's new
partuuid target.  Update grub.texi documentation.  The following table
shows how GRUB_DISABLE_LINUX_UUID, GRUB_DISABLE_LINUX_PARTUUID, and
initramfs detection interact:

Initramfs  GRUB_DISABLE_LINUX_PARTUUID  GRUB_DISABLE_LINUX_UUID  Linux Root
detected   Set                          Set                      ID Method

false      false                        false                    part UUID
false      false                        true                     part UUID
false      true                         false                    dev name
false      true                         true                     dev name
true       false                        false                    fs UUID
true       false                        true                     part UUID
true       true                         false                    fs UUID
true       true                         true                     dev name

Note: GRUB_DISABLE_LINUX_PARTUUID and GRUB_DISABLE_LINUX_UUID equate to
      'false' when unset or set to any value other than 'true'.
      GRUB_DISABLE_LINUX_PARTUUID defaults to 'true'.

Signed-off-by: Nicholas Vinson <nvinson234@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>