Reintroduce the possibility to force an architecture

Among other things, 36459f3 ([archiso] Drop aitab support, 2014-06-28)
removed the possibility to manually set a specific architecture by using
kernel parameters. This, however, is useful, e.g. when installing Arch
Linux on a device that reports itself as i586 but works fine with the
i686 flavor.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>

docs/README.bootparams

@@ -57,6 +57,9 @@ INDEX
                     Default: "75%"
 * dm_snap_prefix=   Set a prefix for device-mapper snapshot node names.
                     Default: "arch"
+* arch=             Force an architecture type (i686 | x86_64).
+                    Do not set it for normal operations.
+                    Default: (architecture of running kernel)
 
 
 ** hooks/archiso_pxe_common