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/dbau1550_el_defconfig b/configs/dbau1550_el_defconfig
index 5970e43..6cd0161 100644
--- a/configs/dbau1550_el_defconfig
+++ b/configs/dbau1550_el_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_LITTLE_ENDIAN=y
 CONFIG_SYS_PROMPT="DbAu1xx0 # "
 # 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_FPGA is not set