Carved out ckreads, disabled at compile-time by default

This moves all ckread-related logic behind the new opt-in compile-time
LFS_CKREADS flag. So in order to use ckreads you need to 1. define
LFS_CKREADS at compile time, and 2. pass LFS_M_CKREADS during
lfsr_mount.

This was always the plan since, even if ckreads worked perfectly, it
adds a significant amount of baggage (stack mostly) to track the
ck context of all reads.

---

This is the first non-trivial opt-in define in littlefs, so more test
framework features!

test.py and build.py now support the optional ifdef attribute, which
makes it easy to indicate a test suite/case should not be compiled when
a feature is missing.

Also interesting to note is the addition of LFS_IFDEF_CKREADS, which
solves several issues (and general ugliness) related to #ifdefs in
expression. For example:

  // does not compile :( (can't embed ifdefs in macros)
  LFS_ASSERT(flags == (
          LFS_M_CKPROGS
              #ifdef LFS_CKREADS
              | LFS_M_CKREADS
              #endif
              ))

  // does compile :)
  LFS_ASSERT(flags == (
          LFS_M_CKPROGS
              | LFS_IFDEF_CKREADS(LFS_M_CKREADS, 0)));

---

This brings us way back down to our pre-ckread levels of code/stack:

                   code          stack
  before-ckreads: 36352           2672
  ckreads:        38060 (+4.7%)   3056 (+14.4%)
  after-ckreads:  36428 (+0.2%)   2680 (+0.3%)

Unfortunately, we do end up with a bit more code cost than where we
started. Mainly due to code moving around to support the ckread
infrastructure:

                   code          stack
  lfsr_bd_readtag:  +52 (+23.2%)    +8 (+10.0%)
  lfsr_rbyd_fetch:  +36 (+5.0%)     +8 (+6.2%, cold)
  lfs_toleb128:     -12 (-25.0%)    -4 (-20.0%, cold)
  total:            +76 (+0.2%)     +8 (+0.3%)

But oh well. Note that some of these changes are good even without
ckreads, such as only parsing the last ecksum tag.

tests/test_ck.toml

@@ -2,6 +2,7 @@
 after = ['test_traversal', 'test_gc', 'test_mount']
 
 
+
 # Test filesystem-level checksum things
 
 # test we can detect at least fully clobbered blocks
@@ -804,6 +805,7 @@ defines.BADBLOCK_BEHAVIOR = [
 ]
 # this should stay inlined
 defines.SIZE = 'BLOCK_SIZE/16'
+ifdef = 'LFS_CKREADS'
 code = '''
     // test all bad bits in the mroot
     for (lfs_size_t i = 0;
@@ -915,6 +917,7 @@ defines.BADBLOCK_BEHAVIOR = [
 ]
 # this should create a single block file
 defines.SIZE = 'BLOCK_SIZE'
+ifdef = 'LFS_CKREADS'
 code = '''
     // first we need to figure out where the data block will actually
     // end up, fortunately our block randomization is intentionally
@@ -1048,6 +1051,7 @@ defines.INLINE_SIZE = 0
 defines.CRYSTAL_THRESH = -1
 defines.FRAGMENT_SIZE = 'BLOCK_SIZE/8'
 defines.SIZE = '2*FRAGMENT_SIZE'
+ifdef = 'LFS_CKREADS'
 code = '''
     // first we need to figure out where the btree block will actually
     // end up, fortunately our block randomization is intentionally