Added LFS_MALLOC/FREE, easier overrides for lfs_malloc/free

Now you can override littlefs's malloc with some simple defines:

  -DLFS_MALLOC=my_malloc
  -DLFS_FREE=my_free

This is probably what most users expected when wanting to override
malloc/free in littlefs, but it hasn't been available, since instead
littlefs provides a file-level override of builtin utils.

The thinking was that there's just too many builtins that could be
overriden, lfs_max/min/alignup/npw2/etc/etc/etc, so allowing users to
just override the util file provides the best flexibility without a ton
of ifdefs.

But it's become clear this is awkward for users that just want to
replace malloc.

Maybe the original goal was too optimistic, maybe there's a better way
to structure this file, or maybe the best API is just a bunch of ifdefs,
I have no idea! This will hopefully continue to evolve.

lfs_util.h

@@ -217,7 +217,9 @@ uint32_t lfs_crc(uint32_t crc, const void *buffer, size_t size);
 // Allocate memory, only used if buffers are not provided to littlefs
 // Note, memory must be 64-bit aligned
 static inline void *lfs_malloc(size_t size) {
-#ifndef LFS_NO_MALLOC
+#if defined(LFS_MALLOC)
+    return LFS_MALLOC(size);
+#elif !defined(LFS_NO_MALLOC)
     return malloc(size);
 #else
     (void)size;
@@ -227,7 +229,9 @@ static inline void *lfs_malloc(size_t size) {
 
 // Deallocate memory, only used if buffers are not provided to littlefs
 static inline void lfs_free(void *p) {
-#ifndef LFS_NO_MALLOC
+#if defined(LFS_FREE)
+    LFS_FREE(p);
+#elif !defined(LFS_NO_MALLOC)
     free(p);
 #else
     (void)p;