Added mount-time LFS_M_FLUSH/SYNC

These simply imply LFS_O_FLUSH/SYNC on all open writable files.
LFS_M_SYNC is equivalent to MS_SYNCHRONOUS in Linux/etc, while
LFS_M_FLUSH is just provided for consistency.

As pure conveniences, these may seem a bit out of scope for littlefs,
except they are _very_ cheap:

           code          stack
  before: 36356           2664
  after:  36356 (+0.0%)   2664 (+0.0%)

Ok, they're not _completely_ free! It just turns out they cost 8 bytes,
and a bit of simplification around flag checking in lfsr_mount saved
8 bytes:

                  code          stack
  before:        36356           2664
  m_flush/sync:  36364 (+0.0%)   2664 (+0.0%)
  mount-no-mask: 36356 (+0.0%)   2664 (+0.0%)

lfs.h

@@ -155,6 +155,8 @@ enum lfs_type {
 #define LFS_M_RDWR               0  // Mount the filesystem as read and write
 #define LFS_M_RDONLY             1  // Mount the filesystem as read only
 #define LFS_M_CKPROGS   0x00000010  // Check progs by reading back progged data
+#define LFS_M_FLUSH     0x00000040  // Open all files with LFS_O_FLUSH
+#define LFS_M_SYNC      0x00000080  // Open all files with LFS_O_SYNC
 
 #define LFS_M_MTREEONLY \
                         0x00010000  // Only traverse the mtree
@@ -173,7 +175,9 @@ enum lfs_type {
 
 // Filesystem info flags
 #define LFS_I_RDONLY    0x00000001  // Filesystem mounted read only
-#define LFS_I_CKPROGS   0x00000010  // Check progs by reading back progged data
+#define LFS_I_CKPROGS   0x00000010  // Filesystem mounted with LFS_M_CKPROGS
+#define LFS_I_FLUSH     0x00000040  // Filesystem mounted with LFS_M_FLUSH
+#define LFS_I_SYNC      0x00000080  // Filesystem mounted with LFS_M_SYNC
 
 #define LFS_I_INCONSISTENT \
                         0x01000000  // Filesystem needs mkconsistent to write