lib: Add a function to split a string into substrings

Some environment variables provide a space-separated list of strings. It
is easier to process these when they are broken out into an array of
strings.

Add a utility function to handle this.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/vsprintf.h

@@ -328,6 +328,30 @@ char *strmhz(char *buf, unsigned long hz);
  */
 void str_to_upper(const char *in, char *out, size_t len);
 
+/**
+ * str_to_list() - Convert a string to a list of string pointers
+ *
+ * Splits a string containing space-delimited substrings into a number of
+ * separate strings, e.g. "this is" becomes {"this", "is", NULL}. If @instr is
+ * empty then this returns just {NULL}. The string should have only a single
+ * space between items, with no leading or trailing spaces.
+ *
+ * @instr: String to process (this is alloced by this function)
+ * Returns: List of string pointers, terminated by NULL. Each entry points to
+ * a string. If @instr is empty, the list consists just of a single NULL entry.
+ * Note that the first entry points to the alloced string.
+ * Returns NULL if out of memory
+ */
+const char **str_to_list(const char *instr);
+
+/**
+ * str_free_list() - Free a string list
+ *
+ * @ptr: String list to free, as created by str_to_list(). This can also be
+ * NULL, in which case the function does nothing
+ */
+void str_free_list(const char **ptr);
+
 /**
  * vsscanf - Unformat a buffer into a list of arguments
  * @inp: input buffer