scripts/gen_build_files.sh: revert to using /bin/sh

People want busybox to build w/o bash, with /bin/sh = ash

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

scripts/gen_build_files.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # bashism:
 # "read -r" without variable name reads line into $REPLY
 # without stripping whitespace.