soc: xtensa: nxp: invoke `west sign` at `west build` time

This aligns `soc/xtensa/nxp_adsp/` with commit
fad2da39 ("intel_adsp: move `west sign` from `west flash` to earlier
`west build`")

The --if-tool-available option preserves backwards-compatibility:
nothing happens if rimage is not found.

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