Add extra build commands that can run after scan/tarball

Commands used in the 'build' section are for actual building that
produces binary output. Some existing stuff will need to move from
'prebuild' to this. See manual for more information.

docs/fdroid.texi

@@ -772,6 +772,11 @@ insert literal commas, or as the last character on a line to join that
 line with the next. It has no special meaning in other contexts; in
 particular, literal backslashes should not be escaped.
 
+Note that nothing should be build during this prebuild phase - scanning
+of the code and building of the source tarball, for example, take place
+after this. For custom actions that actually build things, use 'build'
+instead.
+
 You can use $$name$$ to substitute the path to a referenced srclib - see
 the @code{srclib} directory for details of this.
 
@@ -785,6 +790,15 @@ takes place.
 You can use $$SDK$$, $$NDK$$ and $$MVN3$$ to substitute the paths to the
 android SDK and NDK directories, and maven 3 executable respectively.
 
+@item build=xxxx
+As for 'prebuild', but runs during the actual build phase (but before the
+main ant/maven build). Use this only for actions that do actual building.
+Any prepartion of the source code should be done using 'init' or 'prebuild'.
+
+You can use $$SDK$$, $$NDK$$ and $$MVN3$$ to substitute the paths to the
+android SDK and NDK directories, and maven 3 executable respectively.
+
+
 @item novcheck=yes 
 Don't check that the version name and code in the resulting apk are
 correct by looking at the build output - assume the metadata is