util/testing: Update junit.xml to support coreboot builds

util/testing: Update junit.xml to support coreboot builds

Up to this point, junit.xml has only been used to build tools, as abuild
has handled the coreboot builds.  To add additional tests not covered
by abuild, we need junit.xml to work with bare directories.

This also requires updating the build directory (BLD_DIR) for existing
builds using the junit.xml target.

Change-Id: If6e27e02e25e20f48e5a9372373de6058ca378dd
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/26421
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>