Prefer GRUB as the EFI provider.

The SysOTA+RAUC stack supports GRUB as the point of integration. This is
caused by both RAUC downstream support for GRUB and GRUB's ability to
load the kernel from a squashfs image.

In addition, add TODO about GRUB and microcode updates

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>