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/gr_xc3s_1500_defconfig b/configs/gr_xc3s_1500_defconfig
index 5c1442d..a05b709 100644
--- a/configs/gr_xc3s_1500_defconfig
+++ b/configs/gr_xc3s_1500_defconfig
@@ -1,4 +1,5 @@
 CONFIG_SPARC=y
 CONFIG_TARGET_GR_XC3S_1500=y
 CONFIG_SYS_TEXT_BASE=0x00000000
+# CONFIG_CMD_ELF is not sets
 # CONFIG_CMD_SETEXPR is not set