kmod: set ac_cv_path_DOLT_BASH to /usr/bin/env bash

kmod: set ac_cv_path_DOLT_BASH to /usr/bin/env bash

The shebang's length is usually 128 as defined in /usr/include/linux/binfmts.h:
  #define BINPRM_BUF_SIZE 128

So there would be errors when /path/to/hosttools/bash is longer than 128:

/bin/sh: ./doltcompile: [snip]: bad interpreter: No such file or directory

Set ac_cv_path_DOLT_BASH to "/usr/bin/env bash" to fix the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>