kconfig: add 'shell' built-in function

This accepts a single command to execute.  It returns the standard
output from it.

[Example code]

  config HELLO
          string
          default "$(shell,echo hello world)"

  config Y
          def_bool $(shell,echo y)

[Result]

  $ make -s alldefconfig && tail -n 2 .config
  CONFIG_HELLO="hello world"
  CONFIG_Y=y

Caveat:
Like environments, functions are expanded in the lexer.  You cannot
pass symbols to function arguments.  This is a limitation to simplify
the implementation.  I want to avoid the dynamic function evaluation,
which would introduce much more complexity.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/preprocess.c

@@ -106,8 +106,49 @@ struct function {
 	char *(*func)(int argc, char *argv[]);
 };
 
+static char *do_shell(int argc, char *argv[])
+{
+	FILE *p;
+	char buf[256];
+	char *cmd;
+	size_t nread;
+	int i;
+
+	cmd = argv[0];
+
+	p = popen(cmd, "r");
+	if (!p) {
+		perror(cmd);
+		exit(1);
+	}
+
+	nread = fread(buf, 1, sizeof(buf), p);
+	if (nread == sizeof(buf))
+		nread--;
+
+	/* remove trailing new lines */
+	while (buf[nread - 1] == '\n')
+		nread--;
+
+	buf[nread] = 0;
+
+	/* replace a new line with a space */
+	for (i = 0; i < nread; i++) {
+		if (buf[i] == '\n')
+			buf[i] = ' ';
+	}
+
+	if (pclose(p) == -1) {
+		perror(cmd);
+		exit(1);
+	}
+
+	return xstrdup(buf);
+}
+
 static const struct function function_table[] = {
 	/* Name		MIN	MAX	Function */
+	{ "shell",	1,	1,	do_shell },
 };
 
 #define FUNCTION_MAX_ARGS		16