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

Change-Id: I94ec0a32f5712259d3d0caec2233c992330228e3
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 1104db8328a197c7ccf6959a238277f416a2113a
Original-Signed-off-by: Dan Ehrenberg <dehrenberg@chromium.org>
Original-Change-Id: I50ca5bfdfbef9e084321b2beb1b8d4194ca5af9c
Original-Reviewed-on: https://chromium-review.googlesource.com/231456
Original-Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: http://review.coreboot.org/8733
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/include/string.h

@@ -68,6 +68,13 @@ char *strtok_r(char *str, const char *delim, char **ptr);
 char *strerror(int errnum);
 /** @} */
 
+/**
+ * @defgroup string Unicode functions
+ * @{
+ */
+char *utf16le_to_ascii(uint16_t *utf16_string, int maxlen);
+/** @} */
+
 /**
  * @defgroup string OpenBSD based safe string functions
  * @{