debug-shell: don't be picky, use /bin/sh rather than /bin/bash

In a minimal initramfs, one might not always have the full bash available.
Typically if using busybox.

configure.ac

@@ -276,7 +276,7 @@ AM_CONDITIONAL(HAVE_SELINUX, [test "$have_selinux" = "yes"])
 if test "x${have_selinux}" != xno ; then
         sushell=/sbin/sushell
 else
-        sushell=/bin/bash
+        sushell=/bin/sh
 fi
 AC_SUBST(sushell)