target/: mark Xtensa architecture support as deprecated

Hasn't been updated since it was added in 2009, and requires quite
some special handling in BR.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

target/Config.in.arch

@@ -124,6 +124,7 @@ config BR2_x86_64
 
 config BR2_xtensa
 	bool "Xtensa"
+	depends on BR2_DEPRECATED
 	help
 	  Xtensa is a Tensilica processor IP architecture.
 	  http://en.wikipedia.org/wiki/Xtensa