scripts: move backtrace marker to CT_WORK_DIR

Avoid error when commands in scripts/crosstool-NG.sh fail
before CT_BUILD_DIR is set.

So we need to remove the backtrace marker of a potential previous
build. Previously, it was implicitly removed because we did remove
the directory it was in, which is no longer the case.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[yann.morin.1998@free.fr: remove backtrace marker on start of build]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <20121015094615.GA18673@sig21.net>
Patchwork-Id: 191498

 diff -dur strace-4.5.15.orig/signal.c strace-4.5.15/signal.c

 --- strace-4.5.15.orig/signal.c	2007-01-11 23:08:38.000000000 +0100

 +++ strace-4.5.15/signal.c	2007-07-14 19:24:43.000000000 +0200

 @@ -1440,7 +1440,7 @@

  	  	tcp->u_rval = tcp->u_error = 0;

  		if(tcp->u_arg[0] == 0)

  		  	return 0;

 -		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1]);

 +		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1], 0);

  		return RVAL_NONE | RVAL_STR;

  	}

  	return 0;