abuild: produce valid junit files

If no valid cross compiler is found, the junit file produced
by abuild is invalid, missing the closing </testcase> tag.
This breaks proper reporting in Jenkins of our ARM board at
this moment.

Change-Id: I94bfc7f334d33ceeb53451a7c5125058c1f33bd4
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1992
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Tested-by: build bot (Jenkins)

util/abuild/abuild

@@ -370,6 +370,7 @@ function build_target
 
 			junit "<testcase classname='board' name='$TARCH/$VENDOR/$MAINBOARD' >"
 			junit " <failure type='NoCrossCompiler'>No cross-compiler for $TARCH found</failure>"
+			junit "</testcase>"
 
 			return 0
 		else