dev-manual, ref-manual: replaced "atom-pc" with "genericx86"

Update the manuals to reflect the recent replacement of atom-pc with
genericx86.

(From yocto-docs rev: 8f77006d74b77000f56b64581cb5f6474f53a4f1)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/dev-manual-model.xml

@@ -156,7 +156,7 @@
                     "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-layers'>BSP Layers</ulink>" section in the
                     Yocto Project Board Support Package (BSP) Developer's Guide.</para>
                     <note>Four BSPs exist that are part of the
-                    Yocto Project release: <filename>atom-pc</filename>, <filename>beagleboard</filename>,
+                    Yocto Project release: <filename>genericx86</filename>, <filename>beagleboard</filename>,
                     <filename>mpc8315e</filename>, and <filename>routerstationpro</filename>.
                     The recipes and configurations for these four BSPs are located and dispersed
                     within the <link linkend='source-directory'>Source Directory</link>.

documentation/ref-manual/ref-variables.xml

@@ -2869,7 +2869,7 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
      MACHINE ?= "qemuppc"
      MACHINE ?= "qemux86"
      MACHINE ?= "qemux86-64"
-     MACHINE ?= "atom-pc"
+     MACHINE ?= "genericx86"
      MACHINE ?= "beagleboard"
      MACHINE ?= "mpc8315e-rdb"
      MACHINE ?= "routerstationpro"

documentation/ref-manual/technical-details.xml

@@ -846,7 +846,7 @@
       DEFAULTTUNE = "x86-64-x32"
       baselib = "${@d.getVar('BASE_LIB_tune-' + (d.getVar('DEFAULTTUNE', True) \
          or 'INVALID'), True) or 'lib'}"
-      #MACHINE = "atom-pc"
+      #MACHINE = "genericx86"
       #DEFAULTTUNE = "core2-64-x32"
                     </literallayout></para></listitem>
                 <listitem><para>As usual, use BitBake to build an image that supports the x32 psABI.