[crypto] Fail all operations for the null public-key algorithm

The null crypto algorithms are intended to do nothing: the null digest
algorithm accepts all input and generates a zero-length digest, and
the null cipher algorithm simply copies the input unmodifed to the
output.

The null public-key algorithm currently does nothing successfully.
Unlike the null digest and cipher algorithms, the null public-key
algorithm's methods are never called.

Change the null public-key algorithm to fail all operations, thereby
allowing its methods to be used as stubs by algorithms such as ECDSA
that do not implement all of the possible public-key operations.

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

src/include/ipxe/errfile.h

@@ -449,6 +449,7 @@ FILE_SECBOOT ( PERMITTED );
 #define ERRFILE_efi_cacert	      ( ERRFILE_OTHER | 0x00670000 )
 #define ERRFILE_ecdhe		      ( ERRFILE_OTHER | 0x00680000 )
 #define ERRFILE_ecdsa		      ( ERRFILE_OTHER | 0x00690000 )
+#define ERRFILE_crypto_null	      ( ERRFILE_OTHER | 0x006a0000 )
 
 /** @} */