[config] Change the way to select the CONFIG_SHELL

Transform the single boolean option to a 'choice' selection
with two options: "sh" and "ash".

This is needed to later add "bash" as override.

config/global/build-behave.in

@@ -44,16 +44,46 @@ config USE_PIPES
       Use gcc's option -pipe to use pipes rather than temp files when building
       the toolchain.
 
+choice
+    bool
+    prompt "Shell to use as CONFIG_SHELL"
+    default CONFIG_SHELL_SYSTEM
+
+config CONFIG_SHELL_SH
+    bool
+    prompt "sh (the system shell)"
+    help
+      Use 'sh' as CONFIG_SHELL.
+      
+      ./configure scripts and Makefiles make intensive use of calling
+      sub-shells. This is usually done by calling /bin/sh. /bin/sh ought
+      to be an at-least-POSIX-conformant shell (that is, able to interpret
+      POSIX shell scripts).
+      
+      On many (most?) systems, /bin/sh is a symlink to bash. On some other
+      systems (eg. Ubuntu, latest Debian), /bin/sh points to dash (or ash).
+      bash is a full-featured shell, with many extension to POSIX, but is
+      quite slow (see ection BUGS in the bash man page), while dash is
+      faster, with very few extensions to POSIX.  On the other hand, some
+      ./configure scripts, although written to use /bin/sh, may really
+      require to be run by bash.
+      
+      The default is to use your system's /bin/sh shell. If you want to
+      run faster, you can select to use dash. If you have problems with
+      either the system shell or when using dash, then you can force to
+      use bash.
+      
 config CONFIG_SHELL_ASH
     bool
-    prompt "Use 'ash' as CONFIG_SHELL"
+    prompt "ash"
-    default n
     help
-      Use /bin/ash as the shell used by ./configure scripts.
+      Use 'ash' as CONFIG_SHELL.
       
-      ./configure scripts make intensive use of calling a sub-shell, and the
+      See help for CONFIG_SHELL_SH, above, for more explanations.
-      traditional bash is slow, whereas ash is faster.
 
-      Saying 'y' here should be safe enough, but some components may not
+endchoice
-      accept using a shell other than bash (this is most probably true for
+
-      old versions). If you have trouble, say 'n' here and see if it fixes it.
+config CONFIG_SHELL
+    string
+    default "sh"    if CONFIG_SHELL_SH
+    default "ash"   if CONFIG_SHELL_ASH

scripts/crosstool-NG.sh.in

@@ -63,8 +63,13 @@ CT_DoExecLog DEBUG rm -f "${CT_WORK_DIR}/foo"
 # What's our shell?
 # Will be plain /bin/sh on most systems, except if we have /bin/ash and we
 # _explictly_ required using it
-CT_SHELL="/bin/sh"
+case "${CT_CONFIG_SHELL}" in
-[ "${CT_CONFIG_SHELL_ASH}" = "y" -a -x "/bin/ash" ] && CT_SHELL="/bin/ash"
+    sh)     CT_SHELL="/bin/sh";;
+    ash)    CT_SHELL="/bin/ash";;
+esac
+
+# Check the user is using an existing SHELL to be used by ./configure and Makefiles
+CT_TestOrAbort "The CONFIG_SHELL '${CT_CONFIG_SHELL}' (${CT_SHELL}) is not valid" -f "${CT_SHELL}" -a -x "${CT_SHELL}"
 
 # Create the bin-overide early
 # Contains symlinks to the tools found by ./configure
@@ -452,7 +457,8 @@ if [ -z "${CT_RESTART}" ]; then
 
     # Set the shell to be used by ./configure scripts and by Makefiles (those
     # that support it!).
-    export CONFIG_SHELL="${CT_SHELL}"
+    export CONFIG_SHELL="${CT_SHELL}"   # for ./configure
+    export SHELL="${CT_SHELL}"          # for Makefiles
 
     # And help make go faster
     PARALLELMFLAGS=