[Tizen] gyp: Centralize the dependency on libtzplatform-config.

pkg-config was being used to get libtzplatform-config's flags in both
build/system.gyp and xwalk.gyp. Do that in a single place,
build/system.gyp, instead, to reduce this needless duplication.

Since libtzplatform-config was part of the catch-all "tizen" target in
system.gyp, it has been split into its own separate target so that code
that only needs libtzplatform-config does not end up depending on
several other Tizen libraries as well.