Don't prompt for endianness not suppoted by selected architecture.

Change suggested by Robert P. J. DAY <rpjday@mindspring.com>.

config/target.in

@@ -12,21 +12,35 @@ choice
 config ARCH_ARM
     bool
     prompt "arm"
+    select ARCH_SUPPORTS_BE
+    select ARCH_SUPPORTS_LE
 
 config ARCH_MIPS
     bool
     prompt "mips"
+    select ARCH_SUPPORTS_BE
+    select ARCH_SUPPORTS_LE
 
 config ARCH_x86
     bool
     prompt "x86"
+    select ARCH_SUPPORTS_LE
 
 config ARCH_x86_64
     bool
     prompt "x86_64"
+    select ARCH_SUPPORTS_LE
 
 endchoice
 
+config ARCH_SUPPORTS_BE
+    bool
+    default n
+
+config ARCH_SUPPORTS_LE
+    bool
+    default n
+
 choice
     bool
     prompt "Endianness:"
@@ -34,10 +48,12 @@ choice
 config ARCH_BE
     bool
     prompt "Big endian"
+    depends on ARCH_SUPPORTS_BE
 
 config ARCH_LE
     bool
     prompt "Little endian"
+    depends on ARCH_SUPPORTS_LE
 
 endchoice