libpayload: UTF-16LE to ASCII conversion

This patch adds a simple function to convert a string in UTF-16LE
to ASCII.

TEST=Ran against a string found in a GPT with the intended outcome
BRANCH=none
BUG=none

Signed-off-by: Dan Ehrenberg <dehrenberg@chromium.org>
Change-Id: I50ca5bfdfbef9e084321b2beb1b8d4194ca5af9c
Reviewed-on: https://chromium-review.googlesource.com/231456
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/include/string.h

@@ -67,6 +67,13 @@ char* strtok(char *str, const char *delim);
 char* strtok_r(char *str, const char *delim, char **ptr);
 /** @} */
 
+/**
+ * @defgroup string Unicode functions
+ * @{
+ */
+char *utf16le_to_ascii(uint16_t *utf16_string, int maxlen);
+/** @} */
+
 /**
  * @defgroup string OpenBSD based safe string functions
  * @{