[crypto] Allow for explicit control of external trust sources

[crypto] Allow for explicit control of external trust sources

We currently disable all external trust sources (such as the UEFI
TlsCaCertificate variable) if an explicit TRUST=... parameter is
provided on the build command line.

Define an explicit TRUST_EXT build parameter that can be used to
explicitly disable external trust sources even if no TRUST=...
parameter is provided, or to explicitly enable external trust sources
even if an explicit TRUST=... parameter is provided.  For example:

   # Default trusted root certificate, disable external sources
   make TRUST_EXT=0

   # Explicit trusted root certificate, enable external sources
   make TRUST=custom.crt TRUST_EXT=1

If no TRUST_EXT parameter is specified, then continue to default to
disabling external trust sources if an explicit TRUST=... parameter is
provided, to maintain backwards compatibility with existing build
command lines.

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