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/common/Kconfig b/common/Kconfig
index 2c42b8e..0d44993 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -148,6 +148,12 @@
 	help
 	  Boot an application image from the memory.
 
+config CMD_ELF
+	bool "bootelf, bootvx"
+	default y
+	help
+	  Boot an ELF/vxWorks image from the memory.
+
 config CMD_GO
 	bool "go"
 	default y