EFI stub documentation updates

Move efi-stub.txt out of x86 directory and into common directory
in preparation for adding ARM EFI stub support.
Signed-off-by: NRoy Franz <roy.franz@linaro.org>
Signed-off-by: NMatt Fleming <matt.fleming@intel.com>

arch/x86/Kconfig

@@ -1579,7 +1579,7 @@ config EFI_STUB
           This kernel feature allows a bzImage to be loaded directly
 	  by EFI firmware without the use of a bootloader.
 
-	  See Documentation/x86/efi-stub.txt for more information.
+	  See Documentation/efi-stub.txt for more information.
 
 config SECCOMP
 	def_bool y