Added trace statements through LFS_YES_TRACE

To use, compile and run with LFS_YES_TRACE defined:
make CFLAGS+=-DLFS_YES_TRACE=1 test_format

The name LFS_YES_TRACE was chosen to match the LFS_NO_DEBUG and
LFS_NO_WARN defines for the similar levels of output. The YES is
necessary to avoid a conflict with the actual LFS_TRACE macro that
gets emitting. LFS_TRACE can also be defined directly to provide
a custom trace formatter.

Hopefully having trace statements at the littlefs C API helps
debugging and reproducing issues.

Makefile

@@ -24,6 +24,9 @@ endif
 ifdef WORD
 override CFLAGS += -m$(WORD)
 endif
+ifdef TRACE
+override CFLAGS += -DLFS_YES_TRACE
+endif
 override CFLAGS += -I.
 override CFLAGS += -std=c99 -Wall -pedantic
 override CFLAGS += -Wextra -Wshadow -Wjump-misses-init