examples: allow building all as part of meson build

examples: allow building all as part of meson build

To test building all relevant example applications as part of a build, we
add support for the "all" keyword to be passed to the "examples" build
option. Since not all examples can actually be built on all systems,
we also add support for the "build" option inside the sub-dirs. However,
in case where "all" is not used, and a particular example is requested
to be built, we will error out if building the requested app is not
possible.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Harry van Haaren <harry.van.haaren@intel.com>