Fix #11101: Broken Haiku repository creation.

* Invoking jam from Haiku's checkout directory (instead of the output
  directory) caused the repository creation to fail because of relative
  paths to package files being used. Now we make sure that HAIKU_TOP
  is an absolute path to avoid problems like these.

Jamrules

@@ -1,5 +1,10 @@
 # Haiku Jamrules
 
+# Make sure we have an absolute HAIKU_TOP
+if $(HAIKU_TOP) = "." {
+	HAIKU_TOP = $(PWD) ;
+}
+
 # Main directories used by the build.
 HAIKU_OUTPUT_DIR					?= [ FDirName $(HAIKU_TOP) generated ] ;
 HAIKU_BUILD_OUTPUT_DIR				?= [ FDirName $(HAIKU_OUTPUT_DIR) build ] ;