elf2flt: only allow it to be selected on supported archs

toolchain/elf2flt/Config.in

@@ -1,8 +1,6 @@
-comment "elf2flt"
-
 config BR2_ELF2FLT
 	bool "Enable elf2flt support?"
-	depends !BR2_alpha && !BR2_x86_64
+	depends on BR2_arm || BR2_sh || BR2_sparc
 	default n
 	help
 	  uCLinux uses a Binary Flat format commonly known as BFLT. It