apm821xx: MBL: load kernel/dtb from SATA 0:1 first

apm821xx: MBL: load kernel/dtb from SATA 0:1 first

This remedies an issue with the MBL Duo if both disks are inserted
and contain OpenWrt. kernel and dtb would be loaded from SATA 1:1
while rootfs (/dev/sda2) would be mounted on SATA 0:1.

Such a mix&match would obviously only work if both OpenWrt versions/
builds are identical, and especially fail after sysupgrade upgraded
the system disk on SATA 0:1.

The fallback to SATA 1:1 needs to be kept for MBL Single (only has
SATA 1:1) and MBL Duo with one disk inserted on SATA 1:1. To speed
up booting in those cases, the unneccesarily doubled "sata init"
will only be called once. (In theory it could be omitted completely
since the on-flash boot script already initializes SATA to load the
on-disk boot script.)

Tested on MBL Duo (all possible combination of disks) and MBL Single

Signed-off-by: Freddy Leitner <hello@square.wf>