boot: Tidy local variables in fit_image_load()

Arrange these so that they look a little nicer.

Series-to: concept
Series-version: 2
Cover-letter:
boot: Make fit_image_load() easier to maintain
Before fit_image_load() was created, The code to load kernels, ramdisks
and devicetrees from a FIT was spread around many functions. By
combining most of the code in one place, it became possible to add more
features in a consistent way. The 'loadables' feature much easier to
plumb in, for example.

While fit_image_load() was a substantial advance, it has never been a
svelte function and the passing years have not been entirely kind. With
a few new features on the horizon, this is a good time to improve the
implementation.

This series splits much of the code from fit_image_load() into a number
of smaller functions. Most of the changes are fairly mechanical, with
just a few renames and tweaks here and there.

This should make the function much easier to maintain. It may also
encourage someone to take a look at its callers, which could also use
some attention.
END

Signed-off-by: Simon Glass <sjg@chromium.org>

scripts/build_helper.py

@@ -178,6 +178,7 @@ sct_mnt = /mnt/sct
                 else:
                     tout.warning(f"Disk image '{disk}' not found")
 
+        if args.rand:
             cmd.extend(['-object', 'rng-random,filename=/dev/random,id=rng0',
                         '-device',
                         'virtio-rng-pci,rng=rng0,max-bytes=1024,period=1000'])
@@ -210,6 +211,8 @@ def add_common_args(parser):
     parser.add_argument(
         '-R', '--release', default='24.04.1',
         help='Select OS release version (e.g, 24.04) Default: 24.04.1')
+    parser.add_argument('--rand', '--random', action='store_true',
+                        help='Provide a random-number device')
     parser.add_argument('-s', '--serial-only', action='store_true',
                         help='Run QEMU with serial only (no display)')
     parser.add_argument(