west: sign: add new rimage option --if-tool-available

Moving `west sign` from `west flash` to `west build` for rimage has
multiple advantages (including a bit more consistency with
imgtool). However it makes `west build` fail when rimage is missing.

To avoid forcing every CI and developer who never used it before to
install rimage, make signing optional when passing new `west sign`
option --if-tool-available. A clear warning is printed.

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