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_qspi_defconfig b/configs/xilinx_zynqmp_mini_qspi_defconfig
index d3cc851..b0bc84d 100644
--- a/configs/xilinx_zynqmp_mini_qspi_defconfig
+++ b/configs/xilinx_zynqmp_mini_qspi_defconfig
@@ -11,7 +11,7 @@
 # CONFIG_PSCI_RESET is not set
 # CONFIG_CMD_ZYNQMP is not set
 # CONFIG_EXPERT 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_SPL_TEXT_BASE=0xfffc0000