Build: Enable using HW keys with upstream MCUboot

Modify the build system (by removing restrictions) to allow HW key
usage for image authentication when the 'UPSTREAM' MCUboot repository is
selected as MCUboot v1.6.0 (current default bootloader) already includes
this feature.

Change-Id: Ifa7de6a26febb8b140b409f63fc8c059ed2c3278
Signed-off-by: David Vincze <david.vincze@linaro.org>
3 files changed