Use imported target for res_tool during cross compilation.

[windelbouwman]

Change cmake configuration to use an imported target for the res_tool when cross compiling.

Benefit is that cross compilation is simpler, and the logic for building the res_tool or not is located along with the res_tool source-code, not where the res_tool is used in the corelib.

Remarks welcome, this is useful for me when packaging rtabmap for another architecture.