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-4.5.16.orig/syscall.c strace-4.5.16/syscall.c

--- strace-4.5.16.orig/syscall.c	2006-12-21 23:13:33.000000000 +0100

+++ strace-4.5.16/syscall.c	2007-07-14 19:21:44.000000000 +0200

@@ -1045,6 +1045,15 @@

 		/*

 		 * Note: we only deal with only 32-bit CPUs here.

 		 */

+

+		if (!(tcp->flags & TCB_INSYSCALL) &&

+		    (tcp->flags & TCB_WAITEXECVE)) {

+			/* caught a fake syscall from the execve's exit */

+			tcp->flags &= ~TCB_WAITEXECVE;

+			return 0;

+		}

+

+

 		if (regs.ARM_cpsr & 0x20) {

 			/*

 			 * Get the Thumb-mode system call number