[grub]: use UUID for root partition

[lguohan]

Signed-off-by: Guohan Lu lguohan@gmail.com

- What I did

- How I did it

- How to verify it

                             GNU GRUB  version 2.02

 +----------------------------------------------------------------------------+
 |        insmod part_msdos                                                   |^
 |        insmod ext2                                                         |
 |        linux   /image-uuid.0-dirty-20200418.115911/boot/vmlinuz-4.19.0-6-a\|
 |md64 root=UUID=0530e631-21fa-4707-bbf0-6100631556e3 rw console=tty0 console\|
 |=ttyS0,115200n8 quiet                  net.ifnames=0 biosdevname=0         \|
 |        loop=image-uuid.0-dirty-20200418.115911/fs.squashfs loopfstype=squa\|
 |shfs                                       apparmor=1 security=apparmor var\|
 |log_size=4096 usbcore.autosuspend=-1                                        |
 |        echo    'Loading SONiC-OS OS initial ramdisk ...'                   |
 |        initrd  /image-uuid.0-dirty-20200418.115911/boot/initrd.img-4.19.0-\|
 |6-amd64                                                                     |
 |                                                                            | 
 +----------------------------------------------------------------------------+

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