Updated README to reflect the ability for boards to only have private overlays.

README

     This directory contains board and variant overlays.  When a board is
 configured using "setup_board --board <board>" the following overlays are added
 to the make.conf PORTDIR_OVERLAY if they exist:
 
     src/overlays/overlay-<board>
     src/private-overlays/overlay-<board>-private
 
     When a board is configured using "setup_board --board <board> --variant
 <variant>" or "setup_board --board <board>_<variant>" the following overlays
 are added if they exist:
@@ skipping 26 matching lines @@
 repository names (taken from the repo_name files in the overlays).  This list
 is used to dissambiguate the selection of ebuilds.  When two overlays supply
 the same ebuild the overlay who's repo_name is listed later in the masters list
 will be used.  The masters list also effects the way that portage searches for
 eclasses.  By having the repo_name "chromiumos" as the last entry in the
 masters list portage will correctly find the chromiumos specific eclasses.
 
 
 PRIMARY BOARD OVERLAY
 ---------------------
-    The overlay-<name> directory is special in that it must exist and must
-contain a make.conf and a toolchain.conf.
+    Each board must have a primary board overlay containing make.conf and a
+toolchain.conf file.
+
+    If src/overlays/overlay-<board> exists, it is used as as the primary board
+overlay. If there is no src/overlays/overlay-<board> but there is a
+src/private-overlays/overlay-<board>-private directory, it is used as the
+primary board overlay.
 
     In order to be useful the primary overlay should also contain ebuilds for
-chromeos-base/board-devices and chromeos-base/board-devices-private.  The
-board-devices ebuild is used to override the board-devices ebuild provided by
-the base chromiumos-overlay.  The board-devices-private should be a "null"
-ebuild that is provided so that a private overlay may override it.
+chromeos-base/board-devices. The board-devices ebuild is used to override the
+board-devices ebuild provided by the base chromiumos-overlay.
 
+    If src/overlays/overlay-<board> is the primary board overlay it should
+also contain a chromeos-base/board-devices-private ebuild. The
+board-devices-private ebuild should be a "null" ebuild that is provided so that
+a private overlay may override it.