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/vct_platinumavc_small_defconfig b/configs/vct_platinumavc_small_defconfig
index 0ac2def..8fe82fc 100644
--- a/configs/vct_platinumavc_small_defconfig
+++ b/configs/vct_platinumavc_small_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SYS_PROMPT="$ "
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
+# CONFIG_CMD_ELF is not set
 # CONFIG_CMD_CRC32 is not set
 # CONFIG_CMD_LOADB is not set
 # CONFIG_CMD_LOADS is not set