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/grasshopper_defconfig b/configs/grasshopper_defconfig
index 92a73e2..6c5c9e3 100644
--- a/configs/grasshopper_defconfig
+++ b/configs/grasshopper_defconfig
@@ -5,5 +5,6 @@
 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_FPGA is not set
 # CONFIG_CMD_SETEXPR is not set