Added explicit assert for minimum block size of 128 bytes

There was already an assert for this, but because it included the
underlying equation for the requirement it was too confusing for
users that had no prior knowledge for why the assert could trigger.

The math works out such that 128 bytes is a reasonable minimum
requirement, so I've added that number as an explicit assert.
Hopefully this makes this sort of situation easier to debug.

Note that this requirement would need to be increased to 512 bytes if
block addresses are ever increased to 64-bits. DESIGN.md goes into more
detail why this is.

lfs.c

diff --git a/lfs.c b/lfs.c
index 26280fa..30452aa 100644
--- a/lfs.c
+++ b/lfs.c
@@ -3917,7 +3917,10 @@
     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 ctz pointers
+    // 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
     LFS_ASSERT(4*lfs_npw2(0xffffffff / (lfs->cfg->block_size-2*4))
             <= lfs->cfg->block_size);