dm-verity: add sample systemd separate hash example and doc

Create a wks.in that allows an out-of-the-box build of a bootable
USB image using systemd and the hash data as a separate device or
partition.

A focus here was to ensure we used proper GPT names and GPT types,
and the GPT UUIDs that are based on splitting the root hash.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

docs/dm-verity-systemd-hash-x86-64.txt

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+dm-verity and x86-64 and systemd - separate hash device
+-------------------------------------------------------
+
+Everything said in "dm-verity-systemd-x86-64.txt" applies here.
+However booting under QEMU is not tested - only on real hardware.
+So for your MACHINE you need to choose "genericx86-64".
+
+Also, you'll need to point at the hash specific WKS file:
+
+WKS_FILES += " systemd-bootdisk-dmverity-hash.wks.in"
+
+The fundamental difference is to use a separate device/partition for
+storage of the hash data -- instead of "hiding" it beyond the filesystem
+in what is essentially a 5-10% oversized partition.  This takes any manual
+math calculations of size/offset out of the picture, and uses the kernel's
+natural behaviour of compartmentalizing devices to ensure they are separate.
+
+The example hash.wks file added here essentially adds a hash-only partition
+directly after the filesystem partition.  So the filesystem partition is
+no longer "oversized" and no offsets are needed/used.
+
+Since we are now using multiple partitions, we make a better effort to use
+accepted GPT partition types and UUIDs based on the roothash.  This means
+easier sysadmin level use/debugging based on cfdisk output etc.
+
+Generating the separate root hash image is driven off enabling this:
+   DM_VERITY_SEPARATE_HASH = "1"
+
+Two other variables control the GPT UUIDs - set to x86-64 defaults:
+
+   DM_VERITY_ROOT_GUID ?= "4f68bce3-e8cd-4db1-96e7-fbcaf984b709"
+   DM_VERITY_RHASH_GUID ?= "2c7357ed-ebd2-46d9-aec1-23d437ec2bf5"
+
+See: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/
+
+Finally, the UUIDs (not the "partition types" above) are based off of
+the root node hash value as per the systemd "autodetect" proposed standard.
+These will obviously change with every update/rebuild of the root image.
+
+While not strictly coupled to any functionality at this point in time, it
+does aid in easier debugging, and puts us in alignment with using systemd
+inside the initramfs to replace manual veritysetup like configuration we
+currently do in the initramfs today, should we decide to do so later on.

wic/systemd-bootdisk-dmverity-hash.wks.in

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+# short-description:  Create an EFI disk image with systemd-boot and separate hash dm-verity
+# A dm-verity variant of the regular wks for IA machines. We need to fetch
+# the partition images from the IMGDEPLOYDIR as the rootfs source plugin will
+# not recreate the exact block device corresponding with the hash tree. We must
+# not alter the label or any other setting on the image.
+# Based on OE-core's systemd-bootdisk.wks and meta-security's beaglebone-yocto-verity.wks.in file
+#
+# This .wks only works with the dm-verity-img class and separate hash data. (DM_VERITY_SEPARATE_HASH)
+
+part /boot --source bootimg-efi --sourceparams="loader=systemd-boot,initrd=microcode.cpio" --ondisk sda --label msdos --active --align 1024 --use-uuid
+
+# include the root+hash part with the dynamic hash/UUIDs from the build.
+include ${STAGING_VERITY_DIR}/${IMAGE_BASENAME}.${DM_VERITY_IMAGE_TYPE}.wks.in
+
+# add "console=ttyS0,115200" or whatever you need to the --append="..."
+bootloader --ptable gpt --timeout=5 --append="root=/dev/mapper/rootfs"
+
+part swap --ondisk sda --size 44 --label swap1 --fstype=swap --use-uuid