Make 'build' target phony once again
Make 'build' target phony once again

In commit 5fb4b21 ("Refine deploy target..."), the 'build' target was
made normal, i.e. non-phony, but on further review it does in fact make
sense to declare 'build' phony, such that it is run no matter the status
of the root-level 'build' directory, but for different reasons.

Previously, we had been considering the presence of 'build' directory as
a reasonable proxy for determining whether the `./gradlew build` had
been run. If the directory was present, we considered the 'build' target
up-to-date. If not, then we would re-run `./gradlew build`. This is all
sensible enough, except for the fact that the root-level 'build'
directory has almost nothing to do with the actual output of `./gradlew
build`. Gradle does output 'build' directories, but in the respective
subdirectory for each module of the project. After `./gradlew build` has
been run, we would see a 'desktop/build' directory, a 'seednode/build'
directory and so forth. It just so happens that a root-level 'build'
directory was getting created at all due to idiosyncracies of a
particular Kotlin plugin.

This commit updates the makefile to better respect this reality by:

 - preserving the 'build' target but marking it once again as PHONY

 - introducing new 'seednode/build' and 'desktop/build' targets that
   trigger './gradlew :seednode:build` and ./gradlew :desktop:build`
   commands respectively.

 - making 'build' depend on these two new targets

In light of this realization of flawed thinking about the root-level
build dir, this change also restores `make clean` to calling `./gradlew
clean` instead of `rm -rf build`.

bisq-network/exchange,bitsquare/bitsquare,bisq-network/exchange,bitsquare/bitsquare