[crypto] Add DER image format

Add DER-encoded ASN.1 as an image format.  There is no fixed signature
for DER files.  We treat an image as DER if it comprises a single
valid SEQUENCE object covering the entire length of the image.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/tests.c

@@ -69,3 +69,4 @@ REQUIRE_OBJECT ( pccrc_test );
 REQUIRE_OBJECT ( linebuf_test );
 REQUIRE_OBJECT ( iobuf_test );
 REQUIRE_OBJECT ( bitops_test );
+REQUIRE_OBJECT ( der_test );