stm32mp: stm32prog: solve compilation with CONFIG_FIT_SIGNATURE

When CONFIG_FIT_SIGNATURE is activated, CONFIG_LEGACY_IMAGE_FORMAT
is deactivated and the define IMAGE_FORMAT_LEGACY don't exist with
include/image.h:
  #if defined(CONFIG_LEGACY_IMAGE_FORMAT)
  #define IMAGE_FORMAT_LEGACY	0x01	/* legacy image_header based format */
  #endif

This patch adds the needed check on compilation flag
CONFIG_LEGACY_IMAGE_FORMAT to avoid the compilation error
for command stm32prog:

cmd_stm32prog.c:81:8: error: ‘IMAGE_FORMAT_LEGACY’ undeclared
(first use in this function); did you mean ‘IMAGE_FORMAT_FIT’?
   81 |    if (IMAGE_FORMAT_LEGACY ==
      |        ^~~~~~~~~~~~~~~~~~~
      |        IMAGE_FORMAT_FIT

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>