cmake: run dtc after gen_defines.py and friends

Curently, any and all devicetree syntax errors are being handled by
dtc, because it runs before gen_defines.py. This means that any
improvements we make to devicetree error handling and reporting in
dtlib or edtlib will never be noticed by users that have dtc
installed.

(This also has the unfortunate effect of hiding dtlib error reporting
from me, since my systems all have dtc installed.)

Move dtc to after gen_defines.py, so that initial error handling and
warnings all come from python. If DTC has some additional warnings or
errors to report, let them come last. They will still halt the build
if anything is wrong.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>