internals: Support characters beyond the first unicode plane (WIP)

We used 16-bit variables to store the 'character code' everywhere but
this won't let us represent anything beyond U+FFFF.

This patch changes those variables to a custom type that can be 32 or 16
bits depending on the build, and adjusts numerous internal APIs and
datastructures to match.  This includes:

 * utf8decode() and friends
 * on-screen keyboard
 * font manipulation, caching, rendering, and generation
 * VFAT code parses and generates utf16 dirents
 * WIN32 simulator reads and writes utf16 filenames

Note that this patch doesn't _enable_ >16bit unicode support; a followup
patch will turn that on for appropriate targets.

Known bugs:

  * Native players in 32-bit unicode mode generate mangled filename
    entries if they include UTF16 surrogate codepoints.  Root cause
    is unclear, and may reside in core dircache code.

Needs testing on:

 * windows simulator (16bit+32bit)

Change-Id: I193a00fe2a11a4181ddc82df2d71be52bf00b6e6

firmware/export/cpu.h

@@ -18,6 +18,9 @@
  * KIND, either express or implied.
  *
  ****************************************************************************/
+#ifndef __CPU_H
+#define __CPU_H
+
 #include "config.h"
 
 #if CONFIG_CPU == MCF5249
@@ -80,3 +83,5 @@
 #if CONFIG_CPU == STM32H743
 #include "cpu-stm32h743.h"
 #endif
+
+#endif /* __CPU_H */