[yocto] Allow specifying sub directories for rootfs when using wic -e

[Volker Vogelhuber]

I currently have an image with six different partitions. See the 
following partition configuration:

># bootloader
>part /boot/EFI --source bootimg-efi --sourceparams="loader=systemd-boot" --ondisk mmcblk --fstype=vfat --label boot --active --align 1024 --size 20 --overhead-factor=1.0 --uuid="1EFC2AC2-449B-6ABB-AA63-7EA004446DF1"
>
>#--use-uuid
># primary / recovery image
>part / --source rootfs --rootfs-dir=image --exclude-path opt/something/ opt/else/ opt/somemore/ --ondisk mmcblk --fstype=ext4 --label primary_rootfs --align 1024 --size 768 --overhead-factor=1.0 --uuid="2779D408-1362-AEF5-AEB1-00BF5674C065"
>part /recovery --source rootfs --rootfs-dir=image-recovery --ondisk mmcblk --fstype=ext4 --label recovery_rootfs --align 1024 --size 640 --overhead-factor=1.0 --uuid="528B6F25-5143-47B3-8D12-391820EAF1CF"
>
># additional partitions
>part /opt/something --source rootfs --rootfs-dir=image --rootfs-subdir=opt/something --ondisk mmcblk --fstype=ext4 --label persist --align 1024 --size 64 --overhead-factor=1.0 --use-uuid
>part /opt/else --source rootfs --rootfs-dir=image --rootfs-subdir=opt/else --ondisk mmcblk --fstype=ext4 --label de --align 1024 --size 256 --overhead-factor=1.0 --use-uuid
>part /opt/somemore --source rootfs --rootfs-dir=image --rootfs-subdir=opt/somemore --ondisk mmcblk --fstype=ext4 --label data --align 1024 --size 1700 --overhead-factor=1.0 --use-uuid
>
>bootloader --timeout=0 --ptable gpt --configfile="disk.cfg"

My problem is now that if I use the wic -e option to specify an image 
name as rootfs-dir I can not extract subdirectories from the rootfs 
to different partitions. Or at least I didn't found out a way.
That's why I added a rootfs-subdir option to wic that allows appending 
a rootfs dir to the one received by IMAGE_ROOTFS. I read something 
about spliting should be done on recipe level 
(https://lists.yoctoproject.org/pipermail/yocto/2016-March/029301.html), 
but I couldn't figure out how that should be done and that patch seems 
much easier for me.