image: fit: Show firmware configuration property if present

SPL ATF support requires to have firmware property which should be also
listed by mkimage -l when images is created.

The patch is also using this macro in spl_fit to match keyword.

When image is created:
 Default Configuration: 'config'
 Configuration 0 (config)
  Description:  ATF with full u-boot
  Kernel:       unavailable
  Firmware:     atf
  FDT:          dtb

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