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/pb1000_defconfig b/configs/pb1000_defconfig
index f24cd07..131014d 100644
--- a/configs/pb1000_defconfig
+++ b/configs/pb1000_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="PB1000"
 CONFIG_SYS_PROMPT="Pb1x00 # "
 # CONFIG_CMD_BDI is not set
+# CONFIG_CMD_ELF is not set
 # CONFIG_CMD_RUN is not set
 # CONFIG_CMD_SAVEENV is not set
 # CONFIG_CMD_LOADB is not set