kbuild: add $(BASH) to run scripts with bash-extension

CONFIG_SHELL falls back to sh when bash is not installed on the system,
but nobody is testing such a case since bash is usually installed.
So, shell scripts invoked by CONFIG_SHELL are only tested with bash.

It makes it difficult to test whether the hashbang #!/bin/sh is real.
For example, #!/bin/sh in arch/powerpc/kernel/prom_init_check.sh is
false. (I fixed it up)

Besides, some shell scripts invoked by CONFIG_SHELL use bash-extension
and #!/bin/bash is specified as the hashbang, while CONFIG_SHELL may
not always be set to bash.

Probably, the right thing to do is to introduce BASH, which is bash by
default, and always set CONFIG_SHELL to sh. Replace $(CONFIG_SHELL)
with $(BASH) for bash scripts.

If somebody tries to add bash-extension to a #!/bin/sh script, it will
be caught in testing because /bin/sh is a symlink to dash on some major
distributions.

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

Makefile

@@ -404,9 +404,7 @@ KCONFIG_CONFIG	?= .config
 export KCONFIG_CONFIG
 
 # SHELL used by kbuild
-CONFIG_SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
-	  else if [ -x /bin/bash ]; then echo /bin/bash; \
-	  else echo sh; fi ; fi)
+CONFIG_SHELL := sh
 
 HOST_LFS_CFLAGS := $(shell getconf LFS_CFLAGS 2>/dev/null)
 HOST_LFS_LDFLAGS := $(shell getconf LFS_LDFLAGS 2>/dev/null)
@@ -443,6 +441,7 @@ PYTHON		= python
 PYTHON2		= python2
 PYTHON3		= python3
 CHECK		= sparse
+BASH		= bash
 
 CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
 		  -Wbitwise -Wno-return-void -Wno-unknown-attribute $(CF)
@@ -488,7 +487,7 @@ KBUILD_LDFLAGS :=
 GCC_PLUGINS_CFLAGS :=
 CLANG_FLAGS :=
 
-export ARCH SRCARCH CONFIG_SHELL HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE AS LD CC
+export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE AS LD CC
 export CPP AR NM STRIP OBJCOPY OBJDUMP OBJSIZE PAHOLE LEX YACC AWK INSTALLKERNEL
 export PERL PYTHON PYTHON2 PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
 export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
@@ -1687,7 +1686,7 @@ clean: $(clean-dirs)
 # Generate tags for editors
 # ---------------------------------------------------------------------------
 quiet_cmd_tags = GEN     $@
-      cmd_tags = $(CONFIG_SHELL) $(srctree)/scripts/tags.sh $@
+      cmd_tags = $(BASH) $(srctree)/scripts/tags.sh $@
 
 tags TAGS cscope gtags: FORCE
 	$(call cmd,tags)
@@ -1708,7 +1707,7 @@ versioncheck:
 		| xargs $(PERL) -w $(srctree)/scripts/checkversion.pl
 
 coccicheck:
-	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/$@
+	$(Q)$(BASH) $(srctree)/scripts/$@
 
 namespacecheck:
 	$(PERL) $(srctree)/scripts/namespace.pl