local.conf: add aarch64 to the SDKMACHINE example values

(From meta-yocto rev: 05922833d04205b8b5288b6ccd5f8e32cf1b3bb0)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-poky/conf/local.conf.sample

@@ -112,7 +112,7 @@ PACKAGE_CLASSES ?= "package_rpm"
 # This variable specifies the architecture to build SDK items for and means
 # you can build the SDK packages for architectures other than the machine you are
 # running the build on (i.e. building i686 packages on an x86_64 host).
-# Supported values are i686 and x86_64
+# Supported values are i686, x86_64, aarch64
 #SDKMACHINE ?= "i686"
 
 #