cmd: Convert CONFIG_CMD_ELF to Kconfig

Convert CONFIG_CMD_ELF to Kconfig and tidy up affected boards.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/iocon_defconfig b/configs/iocon_defconfig
index 89e5cf5..63f0c37 100644
--- a/configs/iocon_defconfig
+++ b/configs/iocon_defconfig
@@ -3,4 +3,5 @@
 CONFIG_TARGET_IOCON=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_STOP_STR=" "
+# CONFIG_CMD_ELF is not set
 # CONFIG_CMD_NFS is not set