sha256/512: code shrink. Run-tested.

sha256/512: code shrink. Run-tested.

function                                             old     new   delta
sha512_process_block128                                -    1444   +1444
sha1_process_block64                                   -     542    +542
sha256_process_block64                                 -     529    +529
K512_lo                                                -     320    +320
K256                                                   -     320    +320
init512_lo                                             -      32     +32
init256                                                -      32     +32
sha1_hash                                             99     128     +29
sha256_end                                           160     135     -25
sha1_end                                             189     160     -29
sha512_end                                           237     204     -33
sha256_begin                                          77      44     -33
sha512_begin                                         154      88     -66
sha256_hash                                          338     259     -79
sha512_hash                                          358     262     -96
sha1_compile                                         542       -    -542
sha256_process_block                                 594       -    -594
static.K                                             896       -    -896
sha512_process_block                                1861       -   -1861
------------------------------------------------------------------------------
(add/remove: 7/4 grow/shrink: 1/7 up/down: 3248/-4254)      Total: -1006 bytes
   text    data     bss     dec     hex filename
 808013     468    7856  816337   c74d1 busybox_old
 807007     468    7856  815331   c70e3 busybox_unstripped