configs: SiFive Unmatched: add 'nvme scan' to preboot

configs: SiFive Unmatched: add 'nvme scan' to preboot

Without 'nvme scan' the ESP on the NVMe drive is not found early.
EFI variables cannot be persisted.

    Hit any key to stop autoboot:  0
    Cannot persist EFI variables without system partition
    ** Booting bootflow '<NULL>' with efi_mgr
    Loading Boot0000 'mmc 0' failed
    EFI boot manager: Cannot load any image
    Boot failed (err=-14)
    scanning bus for devices...
    ** Booting bootflow 'nvme#1.blk#1.bootdev.part_1' with efi
    Booting /\EFI\BOOT\BOOTRISCV64.EFI
    error: no suitable video mode found.
    GNU GRUB  version 2.12

With 'nmve scan' booting works as expected.

    Hit any key to stop autoboot:  0
    ** Booting bootflow '<NULL>' with efi_mgr
    Loading Boot0000 'mmc 0' failed
    Loading Boot0001 'nvme 0' failed
    Booting: nvme 1
    error: no suitable video mode found.
    GNU GRUB  version 2.12

Reported by Yuri Zaporozhets <yuriz@vodafonemail.de>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>