Reworked compat flags a bit

Now with a bit more granularity for possibly-future-optional on-disk
data structures:

  LFSR_RCOMPAT_NONSTANDARD  0x0001  ---- ---- ---- ---1 (reserved)
  LFSR_RCOMPAT_MLEAF        0x0002  ---- ---- ---- --1-
  LFSR_RCOMPAT_MSHRUB       0x0004  ---- ---- ---- -1-- (reserved)
  LFSR_RCOMPAT_MTREE        0x0008  ---- ---- ---- 1---
  LFSR_RCOMPAT_BSPROUT      0x0010  ---- ---- ---1 ----
  LFSR_RCOMPAT_BLEAF        0x0020  ---- ---- --1- ----
  LFSR_RCOMPAT_BSHRUB       0x0040  ---- ---- -1-- ----
  LFSR_RCOMPAT_BTREE        0x0080  ---- ---- 1--- ----
  LFSR_RCOMPAT_GRM          0x0100  ---- ---1 ---- ----

  LFSR_WCOMPAT_NONSTANDARD  0x0001  ---- ---- ---- ---1 (reserved)

  LFSR_OCOMPAT_NONSTANDARD  0x0001  ---- ---- ---- ---1 (reserved)

This adds a couple reserved flags:

- LFSR_*COMPAT_NONSTANDARD - This flag will never be set by a standard
  version of littlefs. The idea is to allow implementations with
  non-standard extensions a way to signal potential compatibility issues
  without worrying about future compat flag conflicts.

  This is limited to a single bit, but hey, it's not like it's possible
  to predict all future extensions.

  If a non-standard extension needs more granularity, reservations of
  standard compat flags can always be requested, even if they don't end
  up implemented in standard littlefs. (Though such reservations will
  need a strong motivation, it's not like these flags are free).

- LFSR_RCOMPAT_MSHRUB - In theory littlefs supports a shrubbed mtree,
  where the root is inlined into the mroot. But in practice this turned
  out to be more complicated than it was worth. Still, a future
  implementation may find an mshrub useful, so preserving a compat flag
  for such a case makes sense.

  That being said, I have no plans to add support for mshrubs even in
  the dbg scripts.

  I would like the expected feature-set for debug tools to be
  well-defined, but also conservative. This gets a bit tricky with
  theoretical features like the mshrubs, but until mshrubs are actually
  implemented in littlefs, I would like to consider them non-standard.

  The implication of this is that, while LFSR_RCOMPAT_MSHRUB is
  currently "reserved", it may be repurposed for some other meaning in
  the future.

These changes also rename *COMPATFLAGS -> *COMPAT, and reorder the tags
by decreasing importance. This ordering seems more valuable than the
original intention of making rcompat/wcompat a single bit flip.

Implementation-wise, it's interesting to note the internal-only
LFSR_*COMPAT_OVERFLOW flag. This gets set when out-of-range bits are set
on-disk, and allows us to detect unrepresentable compat flags without
too much extra complexity.

The extra encoding/decoding overhead does add a bit of cost though:

           code          stack
  before: 33944           2880
  after:  34124 (+0.5%)   2880 (+0.0%)

scripts/dbgbmap.py

@@ -14,9 +14,9 @@ TAG_NULL            = 0x0000
 TAG_CONFIG          = 0x0000
 TAG_MAGIC           = 0x0003
 TAG_VERSION         = 0x0004
-TAG_OCOMPATFLAGS    = 0x0005
-TAG_RCOMPATFLAGS    = 0x0006
-TAG_WCOMPATFLAGS    = 0x0007
+TAG_RCOMPAT         = 0x0005
+TAG_WCOMPAT         = 0x0006
+TAG_OCOMPAT         = 0x0007
 TAG_BLOCKSIZE       = 0x0008
 TAG_BLOCKCOUNT      = 0x0009
 TAG_NAMELIMIT       = 0x000a