Several CONFIG_* symbols in config-target.mak are easily computed from just
the target name.  We do not need them in config-target.mak, and can instead
place them in the config_target dictionary only.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>