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 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:  28284 (-25.2%)   1872 (-22.5%)  612 (-3.8%)

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: 23444 (-38.0%)   1736 (-28.1%)  612 (-3.8%)

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%)

Author: geky