[build] Allow trusted root certificates to be specified at build time

[build] Allow trusted root certificates to be specified at build time

Allow trusted root certificates to be specified at build time using
the syntax

  make TRUST=/path/to/certificate1,/path/to/certificate2,...

The build process uses openssl to calculate the SHA-256 fingerprints
of the specified certificates, and adds them to the root certificate
store in rootcert.c.  The certificates can be in any format understood
by openssl.

The certificates may be server certificates or (more usefully) CA
certificates.

If no trusted certificates are specified, then the default "iPXE root
CA" certificate will be used.

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