boot: Introduce the MCUBOOT_ENC_BUILTIN_KEY option

- Introduce the MCUBOOT_ENC_BUILTIN_KEY config option for controlling the
usage of builtin key-encryption keys and replacing the MCUBOOT_HW_KEY
option in the image encryption module to avoid ambiguity as the latter
mentioned option is meant for configuring the handling of the
image verification keys,
- Add encryption build configs to template file.

Change-Id: Ic633d2103fb4e56d9fafdabef1fbddc3244cb795
Signed-off-by: David Vincze <david.vincze@arm.com>