disable mkimage compile, needs portability improvements

target/config/Config.in.tools

@@ -31,12 +31,6 @@ config ADK_HOST_BUILD_LIBTOOL
 	boolean
 	default y
 
-config ADK_HOST_BUILD_U_BOOT
-	boolean
-	default n if ADK_HOST_DARWIN
-	default n if ADK_HOST_CYGWIN
-	default y
-
 # always required, but can be provided by host
 config ADK_HOST_BUILD_BASH
 	boolean