Fix issue 290 for which the default generator was always used by the bootstrapped YCM

[traversaro]

Fix #290 .
This PR contains two fixes:

• Add a missing semicolon ; to the ${_YCM_EP_CMAKE_CACHE_DEFAULT_ARGS} passed to the CMake invocation of the YCM boostrap: without that, the -G<Generator> command was considered as part of the last variable set in _YCM_EP_CMAKE_CACHE_DEFAULT_ARGS, and effectively ignored.
• The previous error for some reason was preventing the use of | as list separtor in the -DCMAKE_PREFIX_PATH:PATH= argument of the CMake invocation to create problems, once fixed the CMake configuration was failing for this reason. As YCM bootstrap does not need to find anything using CMAKE_PREFIX_PATH, the easiest solution is just to remove it from the invocation command.

[drdanz]

Merged, thanks.