init-install-efi.sh: Fix root= specification

init-install-efi.sh: Fix root= specification

Fixes [YOCTO #5237]

The current grub.cfg manipulation depends on an existing root=
parameter. If this doesn't exist, the correct root= parameter will not
be added.

Instead, remove any existing root= parameters and add the correct one
explicitly.

(From OE-Core rev: 14b124122c1b7d10b9a3a96fe4617c6fc1c661c5)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: mihaix.lindner@linux.intel.com
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>