west: sign.py: generate platf.toml from platf.toml.h with cc -E

Allow using the C pre-processor to generate a
`rimage/config/platform.toml` file from a "source"
`rimage/config/platform.toml.h` file.

This is optional and fully backwards compatible.

To use, do not use `-c` and point west sign at a configuration directory
instead or let it use the default `rimage/config/` directory and change
the files there.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>