[crypto] Provide asn1_built() to construct a cursor from a builder

[crypto] Provide asn1_built() to construct a cursor from a builder

Our ASN.1 parsing code uses a struct asn1_cursor, while the object
construction code uses a struct asn1_builder.  These structures are
identical apart from the const modifier applied to the data pointer in
struct asn1_cursor.

Provide asn1_built() to safely typecast a struct asn1_builder to a
struct asn1_cursor, allowing constructed objects to be passed to
functions expecting a struct asn1_cursor.

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