utils: Make serial retarget independent of cmsis-pack-platform

The primary use of iotsdk-serial-retarget CMake target is in examples
and tests. When used by a standalone application however, the current
CMake structure imposes unnecessary constraint on the application
developer. The retargeting lib links to cmsis-pack-platform CMake
target. This is internal to the templates (from which examples and tests
are generated), and not defined for the SDK in general. Lift this
constraint, and make the retargeting lib easier to use in applications.

Update all the templates and link iotsdk-serial-retarget with
cmsis-pack-platform at the application level.

Signed-off-by: Filip Jagodzinski <filip.jagodzinski@arm.com>