u-boot: kernel-fitimage: Fix dependency loop if UBOOT_SIGN_ENABLE and UBOOT_ENV enabled

u-boot: kernel-fitimage: Fix dependency loop if UBOOT_SIGN_ENABLE and UBOOT_ENV enabled

In case both UBOOT_SIGN_ENABLE and UBOOT_ENV are enabled and
kernel-fitimage.bbclass is in use to generate signed kernel
fitImage, there is a circular dependency between uboot-sign
and kernel-fitimage bbclasses . The loop looks like this:

kernel-fitimage.bbclass:
- do_populate_sysroot depends on do_assemble_fitimage
  - do_assemble_fitimage depends on virtual/bootloader:do_populate_sysroot
    - virtual/bootloader:do_populate_sysroot depends on virtual/bootloader:do_install
      => The virtual/bootloader:do_install installs and the
         virtual/bootloader:do_populate_sysroot places into
         sysroot an U-Boot environment script embedded into
         kernel fitImage during do_assemble_fitimage run .

uboot-sign.bbclass:
- DEPENDS on KERNEL_PN, which is really virtual/kernel. More accurately
  - do_deploy depends on do_uboot_assemble_fitimage
  - do_install depends on do_uboot_assemble_fitimage
  - do_uboot_assemble_fitimage depends on virtual/kernel:do_populate_sysroot
    => do_install depends on virtual/kernel:do_populate_sysroot

=> virtual/bootloader:do_install depends on virtual/kernel:do_populate_sysroot
   virtual/kernel:do_populate_sysroot depends on virtual/bootloader:do_install

Attempt to resolve the loop. Pull fitimage configuration options into separate
new configuration file image-fitimage.conf so these configuration options can
be shared by both uboot-sign.bbclass and kernel-fitimage.bbclass, and make use
of mkimage -f auto-conf / mkimage -f auto option to insert /signature node key-*
subnode into U-Boot control DT without depending on the layout of kernel fitImage
itself. This is perfectly valid to do, because the U-Boot /signature node key-*
subnodes 'required' property can contain either of two values, 'conf' or 'image'
to authenticate either selected configuration or all of images when booting the
fitImage.

For details of the U-Boot fitImage signing process, see:
https://docs.u-boot.org/en/latest/usage/fit/signature.html
For details of mkimage -f auto-conf and -f auto, see:
https://manpages.debian.org/experimental/u-boot-tools/mkimage.1.en.html#EXAMPLES

Fixes: 5e12dc911d0c ("u-boot: Rework signing to remove interdependencies")
Reviewed-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>