image: fit: Show information about OS type in firwmare case too

image: fit: Show information about OS type in firwmare case too

SPL ATF implementation requires FIT image with partitions where the one
is Firmware/ATF and another one Firmware/U-Boot. OS field is used for
recording that difference that's why make sense to show values there for
Firmware types.

For example:
 Image 0 (atf)
  Description:  ATF bl31.bin
  Created:      Mon Mar 26 15:58:14 2018
  Type:         Firmware
  Compression:  uncompressed
  Data Size:    51152 Bytes = 49.95 KiB = 0.05 MiB
  Architecture: ARM
  OS:           ARM Trusted Firmware
  Load Address: 0xfffe0000
  Hash algo:    md5
  Hash value:   36a4212bbb698126bf5a248f0f4b5336
 Image 1 (uboot)
  Description:  u-boot.bin
  Created:      Mon Mar 26 15:58:14 2018
  Type:         Firmware
  Compression:  uncompressed
  Data Size:    761216 Bytes = 743.38 KiB = 0.73 MiB
  Architecture: ARM
  OS:           U-Boot
  Load Address: 0x08000000
  Hash algo:    md5
  Hash value:   f22960fe429be72296dc8dc59a47d566

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jun Nie <jun.nie@linaro.org>