[build] Allow a client certificate to be specified at build time

[build] Allow a client certificate to be specified at build time

Allow a client certificate and corresponding private key to be
specified at build time using the syntax

  make CERT=/path/to/certificate KEY=/path/to/key

The build process uses openssl to convert the files into DER format,
and includes them within the client certificate store in
clientcert.c.  The build process will prompt for the private key
password if applicable.

Note that the private key is stored unencrypted, and so the resulting
iPXE binary (and the temporary files created during the build process)
should be treated as being equivalent to an unencrypted private key
file.

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