scripts: fix link-vmlinux.sh bash-ism

While building linux with dash shell:
  LINK    vmlinux
trap: SIGHUP: bad trap
/src/linux-4.0/Makefile:933: recipe for target 'vmlinux' failed
make[1]: *** [vmlinux] Error 1

See the following document for behavior of posix shell trap instruction:
http://pubs.opengroup.org/onlinepubs/000095399/utilities/trap.html

Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sylvain BERTRAND <sylvain.bertrand@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/link-vmlinux.sh

@@ -111,7 +111,6 @@ sortextable()
 }
 
 # Delete output files in case of error
-trap cleanup SIGHUP SIGINT SIGQUIT SIGTERM ERR
 cleanup()
 {
 	rm -f .old_version
@@ -124,6 +123,20 @@ cleanup()
 	rm -f vmlinux.o
 }
 
+on_exit()
+{
+	if [ $? -ne 0 ]; then
+		cleanup
+	fi
+}
+trap on_exit EXIT
+
+on_signals()
+{
+	exit 1
+}
+trap on_signals HUP INT QUIT TERM
+
 #
 #
 # Use "make V=1" to debug this script
@@ -231,7 +244,6 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
 	if ! cmp -s System.map .tmp_System.map; then
 		echo >&2 Inconsistent kallsyms data
 		echo >&2 Try "make KALLSYMS_EXTRA_PASS=1" as a workaround
-		cleanup
 		exit 1
 	fi
 fi