configs: Rename CONFIG_IMAGE_FORMAT_LEGACY to CONFIG_LEGACY_IMAGE_FORMAT

The name CONFIG_LEGACY_IMAGE_FORMAT reads slightly better along with
allowing us to avoid a rather nasty Kbuild/Kconfig issue down the line
with CONFIG_IS_ENABLED(IMAGE_FORMAT_LEGACY).  In a few places outside of
cmd/ switch to using CONFIG_IS_ENABLED() to test what is set.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/xilinx_versal_mini_defconfig b/configs/xilinx_versal_mini_defconfig
index 19fac90..012ba3e 100644
--- a/configs/xilinx_versal_mini_defconfig
+++ b/configs/xilinx_versal_mini_defconfig
@@ -8,7 +8,7 @@
 CONFIG_SYS_MALLOC_LEN=0x2000
 CONFIG_SYS_MEM_RSVD_FOR_MMU=y
 CONFIG_COUNTER_FREQUENCY=2720000
-# CONFIG_IMAGE_FORMAT_LEGACY is not set
+# CONFIG_LEGACY_IMAGE_FORMAT is not set
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_BOARD_EARLY_INIT_R=y