arch: remove sh2, sh3 and sh3eb support

This patch removes the sh2, sh3 and sh3eb support, because the user
base is inexistent, and the Linux support for these architectures is
poor. The sh2a support is preserved, because at least one user
expressed interest in this architecture, and is actually using it:

  http://lists.busybox.net/pipermail/buildroot/2013-April/070399.html



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>