chore(fs): treat '/' as valid driver letter (#6986)

Signed-off-by: Neo Xu <neo.xu1990@gmail.com>

scripts/lv_conf_internal_gen.py

@@ -163,7 +163,7 @@ for line in fin.read().splitlines():
     fout.write(f'{line}\n')
 
 fout.write(
-'''
+r'''
 
 /*----------------------------------
  * End of parsing lv_conf_template.h
@@ -214,6 +214,9 @@ LV_EXPORT_CONST_INT(LV_DRAW_BUF_ALIGN);
     #endif
 #endif
 
+/*Allow only upper case letters and '/'  ('/' is a special case for backward compatibility)*/
+#define LV_FS_IS_VALID_LETTER(l) ((l) == '/' || ((l) >= 'A' && (l) <= 'Z'))
+
 /* If running without lv_conf.h, add typedefs with default value. */
 #ifdef LV_CONF_SKIP
     #if defined(_MSC_VER) && !defined(_CRT_SECURE_NO_WARNINGS)    /*Disable warnings for Visual Studio*/