Replaced lfsr_bd_unprog with an align flag in each prog function

This sort of reverts the addition of lfsr_bd_unprog, but with a slightly
better API. lfsr_bd_unprog was too much of a hack, and isn't really
generalizable. The align flag isn't necessarily any better, but at least
it's the simplest/least-confusing solution available.

And it's net savings, code-wise:

           code          stack          lfs_t
  before: 33690           2608            164
  after:  33678 (-0.0%)   2600 (-0.3%)    160 (-2.4%)

lfs.h

@@ -611,7 +611,6 @@ typedef struct lfs {
         lfs_size_t size;
         uint8_t *buffer;
     } pcache;
-    uint32_t pcksum;
 
     struct lfs_lookahead {
         lfs_block_t start;