configure: don't enable firmware for targets that are not built

This avoids the unfortunate effect of building pc-bios blobs
even for targets the user isn't interested in.

Due to the bi-arch nature of x86 and PPC firmware, check for the
desired target by hand, and don't just look for the compilation target
in $target_list.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed