[crypto] Generalise rsa_parse_integer() to asn1_enter_unsigned()

[crypto] Generalise rsa_parse_integer() to asn1_enter_unsigned()

ECDSA signature values and private keys are fixed-length unsigned
integers modulo N (the group order of the elliptic curve) and are
therefore most naturally represented in ASN.1 using ASN1_OCTET_STRING.

Private key representations do use ASN1_OCTET_STRING, but signature
values tend to use ASN1_INTEGER, which adds no value but does ensure
that the encoding becomes variable-length and requires handling a
pointless extra zero byte if the MSB of the unsigned value happens to
be set.

RSA also makes use of ASN1_INTEGER for modulus and exponent values.
Generalise the existing rsa_parse_integer() to asn1_enter_unsigned()
to allow this code to be reused for ECDSA.

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