Some of the build flags used to be inherited from the CMake target
defining HAL API, i.e. mcu-driver-hal. This is no longer the case as
this simple target has been streamlined to bring the APIs only and all
compile/link-time settings are considered out of scope of
mcu-driver-hal-api.

For example and test templates, set linker flags explicitly, and on per
application basis.

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