grub-efi.bbclass: use GRUB_ROOT rather than APPEND for root device

grub-efi.bbclass: use GRUB_ROOT rather than APPEND for root device

Use APPEND for grub's root device may cause confusion, for example, when
building efi + pcbios, there maybe be two root=/dev/ram0, one of them
would be carried to the installed target, and the target would fail to
boot. Use GRUB_ROOT to fix the problem, and remove SYSLINUX_ROOT from
APPEND will fix the problem.

[YOCTO #9354]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>