[Tizen] enable incremental build for trybot

Since GBS doesn't support incremental build, it takes about 2 hours for every
build, which is not acceptable for trybot. This commit changed the spec file
to skip its source code clean and prepare steps, and instead work directly on
the original xwalk src tree in host system other than chroot system. Thus the
build is greatly speeded up. The major side effect is that the generated souce
rpm package is not intact. For trybot build it's not a big deal. This chanage
is guarded by a parameter passed from GBS command line, so the normal release
build works as usual with no impact.