Added LFS_WCOMPAT_RDONLY and LFS_RCOMPAT_WRONLY

LFS_WCOMPAT_RDONLY seems generally useful for tools that just want to
mark a filesystem is read-only. This is a common flag that exists in
other filesystems (RO_COMPAT_READONLY in ext4 for example).

LFS_RCOMPAT_WRONLY, on the other hand, is a bit more of a joke, but
there could be some niche use cases for it (preventing double mounts?).

Fortunately, these flags require no extra code, and fall out naturally
from our wcompat/rcompat handling.

---

Originally, the idea was to also add LFS_F_RDONLY, to match LFS_M_RDONLY
and set the LFS_WCOMPAT_RDONLY flag during format.

But this doesn't really work with the current API, since lfsr_format
would just give you an empty filesystem you can't write to. Which is a
bit silly.

Maybe we should add something like lfsr_fs_mkrdonly in the future? This
is probably low-priority.

tests/test_mount.toml

@@ -716,6 +716,65 @@ code = '''
     lfsr_unmount(&lfs) => 0;
 '''
 
+# test that we fail to mount rdonly images
+[cases.test_mount_incompat_rdonly]
+in = 'lfs.c'
+code = '''
+    // create a superblock
+    lfs_t lfs;
+    lfsr_format(&lfs, LFS_F_RDWR, CFG) => 0;
+
+    // set the rdonly flag, this prevents writing from a littlefs image
+    //
+    // note we're messing around with internals to do this! this
+    // is not a user API
+    lfsr_mount(&lfs, LFS_M_RDWR, CFG) => 0;
+    uint8_t wcompat_buf[LFSR_WCOMPAT_DSIZE];
+    lfsr_mdir_commit(&lfs, &lfs.mroot, LFSR_RATS(
+            LFSR_RAT(
+                LFSR_TAG_WCOMPAT, 0,
+                LFSR_DATA_WCOMPAT(
+                    LFSR_WCOMPAT_COMPAT
+                        | LFSR_WCOMPAT_RDONLY,
+                    wcompat_buf)))) => 0;
+    lfsr_unmount(&lfs) => 0;
+
+    // mount should now fail
+    lfsr_mount(&lfs, LFS_M_RDWR, CFG) => LFS_ERR_NOTSUP;
+
+    // but we _can_ mount readonly
+    lfsr_mount(&lfs, LFS_M_RDONLY, CFG) => 0;
+    lfsr_unmount(&lfs) => 0;
+'''
+
+# test that we fail to mount wronly images
+[cases.test_mount_incompat_wronly]
+in = 'lfs.c'
+code = '''
+    // create a superblock
+    lfs_t lfs;
+    lfsr_format(&lfs, LFS_F_RDWR, CFG) => 0;
+
+    // set the wronly flag, this prevents reading from a littlefs image
+    //
+    // note we're messing around with internals to do this! this
+    // is not a user API
+    lfsr_mount(&lfs, LFS_M_RDWR, CFG) => 0;
+    uint8_t rcompat_buf[LFSR_RCOMPAT_DSIZE];
+    lfsr_mdir_commit(&lfs, &lfs.mroot, LFSR_RATS(
+            LFSR_RAT(
+                LFSR_TAG_RCOMPAT, 0,
+                LFSR_DATA_RCOMPAT(
+                    LFSR_RCOMPAT_COMPAT
+                        | LFSR_RCOMPAT_WRONLY,
+                    rcompat_buf)))) => 0;
+    lfsr_unmount(&lfs) => 0;
+
+    // mount should now fail
+    lfsr_mount(&lfs, LFS_M_RDWR, CFG) => LFS_ERR_NOTSUP;
+    lfsr_mount(&lfs, LFS_M_RDONLY, CFG) => LFS_ERR_NOTSUP;
+'''
+
 # these are just a bit harder to detect
 [cases.test_mount_incompat_rcompat_overflow]
 defines.OVERFLOW = 72