lib/crypto: Enable more algorithms in cert verification

Right now the code explicitly limits us to sha1,256 hashes with RSA2048
encryption.  But the limitation is artificial since U-Boot supports
a wider range of algorithms.

The internal image_get_[checksum|crypto]_algo() functions expect an
argument in the format of <checksum>,<crypto>.  So let's remove the size
checking and create the needed string on the fly in order to support
more hash/signing combinations.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>