cmake: Add target for generating header files

Before C sources can be compiled any generated header that they
include must be generated. Currently, the target 'offsets_h' happens
to depend directly or indirectly on all generated headers.

This means that to compile safely, one can simply depend on
'offsets_h'. But this is coincidental and might not be true in the
future.

To be able to safely depend on a target that represents all generated
headers being ready we introduce the target
'zephyr_generated_headers'.

Any third-party build scripts can now safely depend on
'zephyr_generated_headers' and be protected from any internal changes
to the build system, like the removal of offsets_h.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>