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>
diff --git a/kernel/Makefile b/kernel/Makefile
index daad787..42557f2 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -128,7 +128,7 @@
 $(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz
 
 quiet_cmd_genikh = CHK     $(obj)/kheaders_data.tar.xz
-      cmd_genikh = $(BASH) $(srctree)/kernel/gen_kheaders.sh $@
+      cmd_genikh = $(CONFIG_SHELL) $(srctree)/kernel/gen_kheaders.sh $@
 $(obj)/kheaders_data.tar.xz: FORCE
 	$(call cmd,genikh)