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_cpci_ax2000_defconfig b/configs/gr_cpci_ax2000_defconfig
index f003d77..ddac3ab 100644
--- a/configs/gr_cpci_ax2000_defconfig
+++ b/configs/gr_cpci_ax2000_defconfig
@@ -1,4 +1,5 @@
 CONFIG_SPARC=y
 CONFIG_TARGET_GR_CPCI_AX2000=y
 CONFIG_SYS_TEXT_BASE=0x00000000
+# CONFIG_CMD_ELF is not set
 # CONFIG_CMD_SETEXPR is not set