Function bootutil_verify_sig() indirectly calls pss_mgf1()
This function has sequence:

    while (count > 0) {
        bootutil_sha_init(&ctx); // !!! Called many times
        bootutil_sha_update(&ctx, hash, PSS_HLEN);
        bootutil_sha_update(&ctx, counter, 4);
        bootutil_sha_finish(&ctx, htmp);
        ...
    }
    bootutil_sha_drop(&ctx); // !!! called only once

For mynewt alternate implementation of hash requires only one bootutil_sha_init() for each bootutil_sha_drop().

Fix would be to put bootutil_sha_drop() just after bootutil_sha_finish() is called.

I'm not sure if moving call to bootutil_sha_drop() few lines about is acceptable or
mynewt code should handle such case on its own.

@utzig what is your opinion on that?