tools/fitimage: make sure dumpimage still works when "@" are detected

tools/fitimage: make sure dumpimage still works when "@" are detected

fit_verify_header fails if it detects unit addresses "@". However, this
will break tools like dumpimage on fit images which worked with previous
versions of the tool (e.g. 2020.04 vs 2021.07). As an example the output
of:
dumpimage -l <fit image>
is:
FIT description: U-Boot fitImage for Linux Distribution
Created:         Thu Jan  1 01:00:00 1970
 Image 0 (kernel@1)
  Description:  Linux kernel
  Created:      Thu Jan  1 01:00:00 1970
  Type:         Kernel Image
  Compression:  gzip compressed
  Data Size:    6442456 Bytes = 6291.46 KiB = 6.14 MiB
  Architecture: AArch64
  OS:           Linux
  Load Address: 0x80080000
  Entry Point:  0x80080000
  Hash algo:    sha256
  Hash value:   ...
 Image 1 (fdt@freescale_fsl-s32g274a-evb.dtb)
  Description:  Flattened Device Tree blob
  Created:      Thu Jan  1 01:00:00 1970
  Type:         Flat Device Tree
  Compression:  uncompressed
  Data Size:    39661 Bytes = 38.73 KiB = 0.04 MiB
  Architecture: AArch64
  Hash algo:    sha256
  Hash value:   ...
 Default Configuration: 'conf@freescale_fsl-s32g274a-evb.dtb'
 Configuration 0 (conf@freescale_fsl-s32g274a-evb.dtb)
  Description:  1 Linux kernel, FDT blob
  Kernel:       kernel@1
  FDT:          fdt@freescale_fsl-s32g274a-evb.dtb
  Hash algo:    sha256
  Hash value:   unavailable

But with newer version it shows:
dumpimage -l <fit image>
GP Header: Size d00dfeed LoadAddr 62f0a4

This commit will output a warning that unit addresses were detected but
will not fail:
dumpimage -l <fit image>
Image contains unit addresses @, this will break signing
...

Signed-off-by: Stefan Eichenberger <eichest@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>