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/atngw100mkii_defconfig b/configs/atngw100mkii_defconfig
index c8d9107..48ea9ba 100644
--- a/configs/atngw100mkii_defconfig
+++ b/configs/atngw100mkii_defconfig
@@ -5,6 +5,7 @@
 CONFIG_AUTOBOOT_PROMPT="Press SPACE to abort autoboot in %d seconds\n"
 CONFIG_AUTOBOOT_DELAY_STR="d"
 CONFIG_AUTOBOOT_STOP_STR=" "
+# CONFIG_CMD_ELF is not set
 # CONFIG_CMD_XIMG is not set
 # CONFIG_CMD_FPGA is not set
 # CONFIG_CMD_SETEXPR is not set