bootutil: Fix crash when bootutil_sha_init() is called in loop

For mynewt hardware implementation of hash functionality
calling bootutil_sha_init() in a loop without call to
bootutil_sha_drop() result in a failure.

In two places where bootutil_sha_drop() was called after
the loop, call is move into the loop to match
calls to bootutil_sha_init().

This will not impact default implementation where calling
bootutil_sha_drop() in a loop does not change anything.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>