board: Add Qemu SH4 r2d configuration

[ Thomas:

   * renamed sh4_defconfig to qemu_sh4_r2d_defconfig, for consistency
     with other Qemu platforms supported
   * renamed board/qemu/sh4 to board/qemu/sh4-r2d
   * minor fixes in the readme.txt
   * remove useless statements in the minimal defconfig
   * switch to a fixed kernel version instead of "same as headers"
]

Signed-off-by: Philippe Reynes <tremyfr@yahoo.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>