Various fix for gc module (#2102)

* fix(gc): remove the unused variables from gc

* fix(gc): correct typo(from _lv_img_defoder_ll to _lv_img_decoder_ll)

* fix(gc): change _lv_drv_ll to _lv_fsdrv_ll

since only instances fo lv_fs_drv_t can be added to this list

* fix(gc): Iterate the root variables only when they are really used

to save the memory space

* fix(image cache): move cache_temp to gc.h

and rename it to _lv_img_cache_single

Kconfig

@@ -274,7 +274,6 @@ menu "LVGL configuration"
                 With complex image decoders (e.g. PNG or JPG) caching can
                 save the continuous open/decode of images.
                 However the opened images might consume additional RAM.
-                LV_IMG_CACHE_DEF_SIZE must be >= 1
     endmenu
 
     menu "Compiler Settings"