Added LFS3_2BONLY for a small 2-block configuration

Like LFS3_RDONLY and LFS3_KVONLY, LFS3_2BONLY opts-out of all of the
logic necessary for filesystems larger than 2-blocks (the mimimum size
of a mutable littlefs image).

This has potential for some pretty big savings:

- No block allocation
- No lookahead buffer
- No btrees (but yes bshrubs)
- No bptrs
- No mtree traversal

Which is I guess ~1/4 of the codebase:

            code           stack           ctx
  default: 37836            2416           636
  2bonly:  27704 (-26.8%)   1872 (-22.5%)  592 (-6.9%)

This can be combined with LFS3_KVONLY for a small key-value store
compatible with the full littlefs driver:

                  code           stack           ctx
  default:       37836            2416           636
  kvonly:        30792 (-18.6%)   2168 (-10.3%)  636 (+0.0%)
  kvonly+2bonly: 22900 (-39.5%)   1736 (-28.1%)  592 (-6.9%)

It may be possible to optimize this further, but, as is the case with
LFS3_KVONLY, balancing config-specific optimization vs maintainability
is tricky.

---

I'm not sure why, but this also reduced the default build's size a bit.
Compiler noise?

           code          stack          ctx
  before: 37860           2416          636
  after:  37836 (-0.1%)   2416 (+0.0%)  636 (+0.0%)

lfs3_util.h

@@ -56,6 +56,9 @@
 #ifdef LFS3_YES_KVONLY
 #define LFS3_KVONLY
 #endif
+#ifdef LFS3_YES_2BONLY
+#define LFS3_2BONLY
+#endif
 #ifdef LFS3_YES_REVDBG
 #define LFS3_REVDBG
 #endif
@@ -206,6 +209,12 @@
 #define LFS3_IFDEF_KVONLY(a, b) (b)
 #endif
 
+#ifdef LFS3_2BONLY
+#define LFS3_IFDEF_2BONLY(a, b) (a)
+#else
+#define LFS3_IFDEF_2BONLY(a, b) (b)
+#endif
+
 #ifdef LFS3_REVDBG
 #define LFS3_IFDEF_REVDBG(a, b) (a)
 #else