cmake: ninja usage build target

Fixed usage build target.

Previously, `ninja usage` would print:
Build flags:

  ninja VERBOSE=1 [targets] verbose build

This is clearly wrong, as VERBOSE=1 is for make targets.

To support both ninja based builds and make based build systems, the
CMAKE_MAKE_PROGRAM is now exported from the calling CMake / build
instance, and the verbose flag is set depending on whether ninja or
make is used as build program.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>