This commit modifies the DTS cmake module to capture `stderr`
output of the `gen_defines.py` script and `dtc` program during
their execution. The messages can then be printed as CMake
`message`s, which improves QoL when debugging device tree
errors, and reduces the risk of introducing malformed DTS,
as the warning/error messages are made much more visible.
Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>
