Add support for FrSky OSD

- Add displayWriteFontCharacter() for font writing, removing all max7456
specific code.
- Add displayIsReady() for asynchronous display initialization
- Add displayBeginTransaction()/displayCommitTransaction() for display
transactions, which allow performing complex drawing operations without
flickering
- Add displayGetCanvas(), which retrieves the canvas associated with a
display (if it has it)
- Add canvas implementation for pixel based access for a display
- Add FrSkyOSD driver and displayPort driver
- Enable FrSkyOSD driver for targets with flash > 256
- Rename max7456_symbols.h to osd_symbols.h

src/test/unit/link_quality_unittest.cc

@@ -33,7 +33,7 @@ extern "C" {
     #include "common/printf.h"
     #include "common/streambuf.h"
 
-    #include "drivers/max7456_symbols.h"
+    #include "drivers/osd_symbols.h"
     #include "drivers/persistent.h"
     #include "drivers/serial.h"
     #include "drivers/system.h"