Make "/default" an alias - add real names too

Make the built-in FileSystem::get_default_instance() implementation
instantiate storage as "flash" or "sd", with "default" as an alias.

This will aid interworking between simple and advanced filesystem code
on various platforms. The expectation is that the names "sd" or "flash"
will be always available if the device is available and configured,
regardless of what "default" represents.

Author: kjbracey

features/storage/system_storage/SystemStorage.cpp

@@ -80,15 +80,17 @@ MBED_WEAK FileSystem *FileSystem::get_default_instance()
 {
 #if COMPONENT_SPIF || COMPONENT_DATAFLASH
 
-    static LittleFileSystem default_fs("default", BlockDevice::get_default_instance());
+    static LittleFileSystem flash("flash", BlockDevice::get_default_instance());
+    flash.set_as_default();
 
-    return &default_fs;
+    return &flash;
 
 #elif COMPONENT_SD
 
-    static FATFileSystem default_fs("default", BlockDevice::get_default_instance());
+    static FATFileSystem sdcard("sd", BlockDevice::get_default_instance());
+    sdcard.set_as_default();
 
-    return &default_fs;
+    return &sdcard;
 
 #else