kheaders: remove the last bashism to allow sh to run it

'pushd' ... 'popd' is the last bash-specific code in this script.
One way to avoid it is to run the code in a sub-shell.

With that addressed, you can run this script with sh.

I replaced $(BASH) with $(CONFIG_SHELL), and I changed the hashbang
to #!/bin/sh.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

kernel/gen_kheaders.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 
 # This script generates an archive consisting of kernel headers
@@ -57,11 +57,12 @@ rm -rf $cpio_dir
 mkdir $cpio_dir
 
 if [ "$building_out_of_srctree" ]; then
-	pushd $srctree > /dev/null
-	for f in $dir_list
-		do find "$f" -name "*.h";
-	done | cpio --quiet -pd $cpio_dir
-	popd > /dev/null
+	(
+		cd $srctree
+		for f in $dir_list
+			do find "$f" -name "*.h";
+		done | cpio --quiet -pd $cpio_dir
+	)
 fi
 
 # The second CPIO can complain if files already exist which can happen with out