dropbear: add 0.53.1

dropbear: add 0.53.1

* 0.53.1 brings some improvements over 0.52 which was released
more than 2 years ago. For more details, check the changelog here :
http://matt.ucc.asn.au/dropbear/CHANGES

* we are using DROPBEAR_SMALL_CODE as a default which saves ~50kB
but makes the symmetrical ciphers (AES, DES etc) slower (perhaps by 50%)
if you need performance you can remove the line in do_configure_prepend

Signed-off-by: Eric Bénard <eric@eukrea.com>
Acked-by: Khem Raj <raj.khem@gmail.com>