README: fix DISTRO = "" reference

DISTRO should be set to "nodistro" for distro-less configurations since
empty string results in empty items ending up in OVERRIDES leading to
unexpected behaviour with FILESPATH.

(From OE-Core rev: 25436b6813b6245faeb981cf93a0c32b871dd8d3)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

README

@@ -22,7 +22,7 @@ reference manual which can be found at:
 
 OpenEmbedded-Core is a layer containing the core metadata for current versions
 of OpenEmbedded. It is distro-less (can build a functional image with
-DISTRO = "") and contains only emulated machine support.
+DISTRO = "nodistro") and contains only emulated machine support.
 
 For information about OpenEmbedded, see the OpenEmbedded website:
     http://www.openembedded.org/