utils/test-pkg: show default build dir in help test

Avoid the "Where did it put my builds?" step.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

utils/test-pkg

@@ -257,6 +257,7 @@ Options:
 
     -d DIR, --build-dir DIR
         Do the builds in directory DIR, one sub-dir per toolchain.
+        If not specified, defaults to \${HOME}/br-test-pkg
 
     -p PKG, --package PKG
         Test-build the package PKG, by running 'make PKG'; if not specified,