oniro-wic: refrain from populating slot B with a copy of slot A

This saves half of the space used by compressed disk artifacts.

I ran into the issue when creating grubenv recovery logic. To my surprise
the execution flow ended up in the case where both slots are available
and timestamp on the kernel is used to break the tie.

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

meta-oniro-core/conf/distro/include/oniro-wic.inc

@@ -48,7 +48,7 @@ WIC_ROOTA_PARTITION ?= "part / --source rootfs --use-uuid --fstype=${ROOT_FSTYPE
                         ${@bb.utils.contains('ROOT_FSTYPE', 'ext4', '--mkfs-extraopts \'-T default\'', '', d)} \
                         --align 4096 --fixed-size ${ROOTA_PARTITION_SIZE} \
                         ${WIC_ROOTA_PARTITION_EXTRA_ARGS}"
-WIC_ROOTB_PARTITION ?= "part / --source rootfs --use-uuid --fstype=${ROOT_FSTYPE} \
+WIC_ROOTB_PARTITION ?= "part / --source empty --use-uuid --fstype=${ROOT_FSTYPE} \
                         ${@bb.utils.contains('ROOT_FSTYPE', 'ext4', '--mkfs-extraopts \'-T default\'', '', d)} \
                         --align 4096 --fixed-size ${ROOTB_PARTITION_SIZE} \
                         ${WIC_ROOTB_PARTITION_EXTRA_ARGS}"