wic: do not ignore ROOTFS_SIZE if the rootfs is modified

wic: do not ignore ROOTFS_SIZE if the rootfs is modified

If the *.wks file contains a "--source rootfs" then
lib/wic/plugins/source/rootfs.py will be invoked to generate (what is assumed
to be) the rootfs partition. If the rootfs partition needs to be tweaked or
modified, the "rootfs.py" plugin will make a copy of the filesystem and then
perform the changes on that copy. In other words, if the "--source rootfs"
line of the *.wks file also contains any of:

	--exclude-path
	--include-path
	--change-directory
	--use-label (i.e. modify etc/fstab)

then the rootfs will be copied first, then the copy is modified.

If, for example, the unmodified IMAGE_ROOTFS is:

	.../tmp/work/qemuarm64_secureboot-oe-linux/core-image-base/1.0/rootfs

then the copy would be made at:

	.../tmp/work/qemuarm64_secureboot-oe-linux/core-image-base/1.0/tmp-wic/rootfs${LINENO}

where ${LINENO} is the line number where this "--source rootfs" line appears
in the *wks file.

When it comes time to make an actual partition of a specific filesystem type,
lib/wic/partition.py::prepare_rootfs() is called. It is in this function that
wic figures out if any extra size needs to be added. The bitbake variable used
to specify the ultimate rootfs size is ROOTFS_SIZE, and since this variable is
only valid for the rootfs (and not any other partitions), the code also
verifies that the partition being created is ${IMAGE_ROOTFS}:

	rsize_bb = get_bitbake_var('ROOTFS_SIZE')
	rdir = get_bitbake_var('IMAGE_ROOTFS')
	if rsize_bb and rdir == rootfs_dir:
		<use rsize_bb>
	else:
		<calculate the partition size using "du -ks $p">

As noted above, if lib/wic/plugins/source/rootfs.py has made a copy, then the
"rdir == rootfs_dir" clause will fail and the code will assume this partition
is not a rootfs since the strings do not compare equal.

Therefore, in order to determine if this is a rootfs, retain the existing
"rdir == rootfs_dir" comparison, but also add another one to check whether or
not this is a wic-generated copy of the rootfs.

STEPS TO REPRODUCE:
	- start with the following *wks file:
		bootloader --ptable gpt
		part /boot --size=100M --active --fstype=ext4 --label boot
		part /     --source rootfs      --fstype=ext4 --label root
	- and the following extra variable in conf/local.conf:
		IMAGE_ROOTFS_EXTRA_SPACE = "500000"
	- build an image
	- run it in qemu
		$ runqemu slirp nographic serial
	- verify the root partition has extra space:
		root@qemuarm64-secureboot:~# df -h
		Filesystem                Size      Used Available Use% Mounted on
		/dev/root               721.5M     67.4M    600.6M  10% /
		devtmpfs                477.7M         0    477.7M   0% /dev
		tmpfs                    40.0K         0     40.0K   0% /mnt
		tmpfs                   489.3M     92.0K    489.2M   0% /run
		tmpfs                   489.3M     68.0K    489.2M   0% /var/volatile
		/dev/vda1               120.4M     19.9M     91.4M  18% /boot
	- modify the "/" line of the *wks file to be:
		part /     --source rootfs      --fstype=ext4 --label root --exclude-path boot/
	- build image

	when it fails:
		root@qemuarm64-secureboot:~# df -h
		Filesystem                Size      Used Available Use% Mounted on
		/dev/root                73.4M     41.9M     25.8M  62% /
		devtmpfs                477.7M         0    477.7M   0% /dev
		tmpfs                    40.0K         0     40.0K   0% /mnt
		tmpfs                   489.3M     92.0K    489.2M   0% /run
		tmpfs                   489.3M     68.0K    489.2M   0% /var/volatile
		/dev/vda1               120.4M     19.9M     91.4M  18% /boot

	after this fix:
		root@qemuarm64-secureboot:~# df -h
		Filesystem                Size      Used Available Use% Mounted on
		/dev/root               721.5M     47.4M    620.6M   7% /
		devtmpfs                477.7M         0    477.7M   0% /dev
		tmpfs                    40.0K         0     40.0K   0% /mnt
		tmpfs                   489.3M     92.0K    489.2M   0% /run
		tmpfs                   489.3M     68.0K    489.2M   0% /var/volatile
		/dev/vda1               120.4M     19.9M     91.4M  18% /boot

Doing the math we see that the /boot partition is ~20MB and in the first image
the / partition contains this ~20MB in addition to the rest of the rootfs.
This ~20MB is completely wasted since it is used in the / partition, but then
the /boot partition is mounted on top of it, making the /boot directory of /
inaccessible. After the fix the / partition has an additional ~20MB since the
/boot portion is excluded.

Fixes [YOCTO #15555]

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>