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_zynqmp_mini_defconfig b/configs/xilinx_zynqmp_mini_defconfig
index aa9dd23..93fa7d8 100644
--- a/configs/xilinx_zynqmp_mini_defconfig
+++ b/configs/xilinx_zynqmp_mini_defconfig
@@ -7,7 +7,7 @@
 CONFIG_SYS_MEM_RSVD_FOR_MMU=y
 CONFIG_ZYNQMP_PSU_INIT_ENABLED=y
 # CONFIG_CMD_ZYNQMP is not set
-# CONFIG_IMAGE_FORMAT_LEGACY is not set
+# CONFIG_LEGACY_IMAGE_FORMAT is not set
 # CONFIG_BOARD_LATE_INIT is not set
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_CMDLINE_EDITING is not set