build,specfile: Implement support for build flags

Currently, it supports two flags: noDebug, noDelta.

The usage is simple:

    <Package>
        ...
        <BuildFlags>
            <Flag>noDebug</Flag>
            <Flag>noDelta</Flag>
        </BuildFlags>
        ...
    </Package>

noDebug flag is used to disable building of a debug package for this
package tag.

noDelta flag is used to disable building of delta packages
automatically. It doesn't change the behaviour of "pisi delta" command.

pisi/operations/build.py

@@ -1001,6 +1001,9 @@ class Builder:
         if ctx.config.values.build.generatedebug:
             debug_packages = []
             for package in self.spec.packages:
+                if "noDebug" in package.buildFlags:
+                    continue
+
                 obj = self.generate_debug_package_object(package)
                 if obj:
                     debug_packages.append(obj)
@@ -1126,7 +1129,11 @@ class Builder:
             # FIXME Remove this hack
             pkg.metadata.package.debug_package = package.debug_package
 
-            delta_packages = self.build_delta_packages(pkg)
+            if "noDelta" not in package.buildFlags:
+                delta_packages = self.build_delta_packages(pkg)
+            else:
+                delta_packages = []
+
             self.delta_map[name] = delta_packages
 
             pkg.close()

pisi/specfile.py

@@ -232,6 +232,7 @@ class Package:
     t_PartOf = [autoxml.String, autoxml.optional]
     t_License = [ [autoxml.String], autoxml.optional]
     t_Icon = [ autoxml.String, autoxml.optional]
+    t_BuildFlags = [[autoxml.String], autoxml.optional, "BuildFlags/Flag"]
     t_BuildType = [ autoxml.String, autoxml.optional ]
     t_BuildDependencies = [[pisi.dependency.Dependency], autoxml.optional]
     t_PackageDependencies = [ [pisi.dependency.Dependency], autoxml.optional, "RuntimeDependencies/Dependency"]