Added an assert for truthy-preserving bool conversions

This has caught enough people that an explicit assert is warranted.
How littlefs, a c99 project, should be integrated with c89 projects
is still an open question, but no one deserves to debug this sort of
undetected casting issue.

Found by johnernberg and XinStellaris

lfs.c

@@ -3905,6 +3905,12 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     lfs->cfg = cfg;
     int err = 0;
 
+    // check that bool is a truthy-preserving type
+    //
+    // note the most common reason for this failure is a before-c99 compiler,
+    // which littlefs currently does not support
+    LFS_ASSERT((bool)0x80000000);
+
     // validate that the lfs-cfg sizes were initiated properly before
     // performing any arithmetic logics with them
     LFS_ASSERT(lfs->cfg->read_size != 0);
@@ -3917,10 +3923,7 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     LFS_ASSERT(lfs->cfg->cache_size % lfs->cfg->prog_size == 0);
     LFS_ASSERT(lfs->cfg->block_size % lfs->cfg->cache_size == 0);
 
-    // check that the block size is large enough to fit all ctz pointers
-    LFS_ASSERT(lfs->cfg->block_size >= 128);
-    // this is the exact calculation for all ctz pointers, if this fails
-    // and the simpler assert above does not, math must be broken
+    // check that the block size is large enough to fit ctz pointers
     LFS_ASSERT(4*lfs_npw2(0xffffffff / (lfs->cfg->block_size-2*4))
             <= lfs->cfg->block_size);