meson: avoid building executables that won't be installed

meson: avoid building executables that won't be installed

When executable() or custom_target() has install: that is conditional as is
false (i.e. not install:true), it won't be built by default. (build_by_default:
defaults to install:). But if that program is added to public_programs, it will
be build by default because it is pulled in by the test, effectively defeating
the disablement.

While at it, make 'ukify' follow the same pattern as 'kernel-install'.
They will be used later together.