Added thread-safe build+size reporting to CI

This removes quite a bit of extra code needed to entertwine the
LFS_TRACE calls into the original funcions.

Also changed temporary return type to match API declaration where
necessary.

.travis.yml

@@ -208,6 +208,22 @@ jobs:
     script:
       - make test TFLAGS+="-k --valgrind"
 
+  # test compilation in thread-safe mode
+  - stage: test
+    env:
+      - NAME=littlefs-threadsafe
+      - CC="arm-linux-gnueabi-gcc --static -mthumb"
+      - CFLAGS="-Werror -DLFS_THREADSAFE"
+    if: branch !~ -prefix$
+    install:
+      - *install-common
+      - sudo apt-get install
+            gcc-arm-linux-gnueabi
+            libc6-dev-armel-cross
+      - arm-linux-gnueabi-gcc --version
+    # report-size will compile littlefs and report the size
+    script: [*report-size]
+
   # self-host with littlefs-fuse for fuzz test
   - stage: test
     env:

lfs.h

@@ -9,6 +9,7 @@
 
 #include <stdint.h>
 #include <stdbool.h>
+#include "lfs_util.h"
 
 #ifdef __cplusplus
 extern "C"
@@ -174,6 +175,16 @@ struct lfs_config {
     // are propogated to the user.
     int (*sync)(const struct lfs_config *c);
 
+#ifdef LFS_THREADSAFE
+    // Lock the underlying block device. Negative error codes
+    // are propogated to the user.
+    int (*lock)(const struct lfs_config *c);
+
+    // Unlock the underlying block device. Negative error codes
+    // are propogated to the user.
+    int (*unlock)(const struct lfs_config *c);
+#endif
+
     // Minimum size of a block read. All read operations will be a
     // multiple of this value.
     lfs_size_t read_size;